rbd-target-api IOError on adding second gateway

I just wiped my configuration (deleted gateway.conf from the pool and rebooted the hosts) after already having the iSCSI Gateway working without any issues.

I followed the same steps as I did before - on my first node (node1) I created a target, added the node itself as a gateway and then tried adding a second node (node2) as gateway.
It however now errors out every time (even after multiple reboots and deleting gateway.conf from the pool) with the following message:

```
/iscsi-target...sigw/gateways> create node1 192.168.44.205
Adding gateway, sync'ing 0 disk(s) and 0 client(s)
ok
/iscsi-target...sigw/gateways> create node2 192.168.44.206
Adding gateway, sync'ing 0 disk(s) and 0 client(s)
ValueError: No JSON object could be decoded
```

rbd-target-api on node2 shows the following:

```
Aug 24 03:39:44 node2.ceph.xxxx.com rbd-target-api[20832]: Started the configuration object watcher
Aug 24 03:39:44 node2.ceph.xxxx.com rbd-target-api[20832]: Checking for config object changes every 1s
Aug 24 03:39:44 node2.ceph.xxxx.com rbd-target-api[20832]: * Running on http://0.0.0.0:5000/
Aug 24 03:39:57 node2.ceph.xxxx.com rbd-target-api[20832]: Change detected - internal 1 / xattr 2 refreshing
Aug 24 03:40:02 node2.ceph.xxxx.com rbd-target-api[20832]: 192.168.44.205 - - [24/Aug/2017 03:40:02] "GET /api/sysinfo/ipv4_addresses HTTP/1.1" 200 -
Aug 24 03:40:02 node2.ceph.xxxx.com rbd-target-api[20832]: 192.168.44.205 - - [24/Aug/2017 03:40:02] "GET /api/sysinfo/checkconf HTTP/1.1" 200 -
Aug 24 03:40:02 node2.ceph.xxxx.com rbd-target-api[20832]: 192.168.44.205 - - [24/Aug/2017 03:40:02] "GET /api/sysinfo/checkversions HTTP/1.1" 200 -
Aug 24 03:40:02 node2.ceph.xxxx.com rbd-target-api[20832]: 192.168.44.205 - - [24/Aug/2017 03:40:02] "PUT /api/_gateway/node2 HTTP/1.1" 500 -
Aug 24 03:40:02 node2.ceph.xxxx.com rbd-target-api[20832]: Traceback (most recent call last):
Aug 24 03:40:02 node2.ceph.xxxx.com rbd-target-api[20832]: File "/usr/lib/python2.7/site-packages/flask/app.py", line 1836, in __call__
Aug 24 03:40:02 node2.ceph.xxxx.com rbd-target-api[20832]: return self.wsgi_app(environ, start_response)
Aug 24 03:40:02 node2.ceph.xxxx.com rbd-target-api[20832]: File "/usr/lib/python2.7/site-packages/flask/app.py", line 1820, in wsgi_app
Aug 24 03:40:02 node2.ceph.xxxx.com rbd-target-api[20832]: response = self.make_response(self.handle_exception(e))
Aug 24 03:40:02 node2.ceph.xxxx.com rbd-target-api[20832]: File "/usr/lib/python2.7/site-packages/flask/app.py", line 1403, in handle_exception
Aug 24 03:40:02 node2.ceph.xxxx.com rbd-target-api[20832]: reraise(exc_type, exc_value, tb)
Aug 24 03:40:02 node2.ceph.xxxx.com rbd-target-api[20832]: File "/usr/lib/python2.7/site-packages/flask/app.py", line 1817, in wsgi_app
Aug 24 03:40:02 node2.ceph.xxxx.com rbd-target-api[20832]: response = self.full_dispatch_request()
Aug 24 03:40:02 node2.ceph.xxxx.com rbd-target-api[20832]: File "/usr/lib/python2.7/site-packages/flask/app.py", line 1477, in full_dispatch_request
Aug 24 03:40:02 node2.ceph.xxxx.com rbd-target-api[20832]: rv = self.handle_user_exception(e)
Aug 24 03:40:02 node2.ceph.xxxx.com rbd-target-api[20832]: File "/usr/lib/python2.7/site-packages/flask/app.py", line 1381, in handle_user_exception
Aug 24 03:40:02 node2.ceph.xxxx.com rbd-target-api[20832]: reraise(exc_type, exc_value, tb)
Aug 24 03:40:02 node2.ceph.xxxx.com rbd-target-api[20832]: File "/usr/lib/python2.7/site-packages/flask/app.py", line 1475, in full_dispatch_request
Aug 24 03:40:02 node2.ceph.xxxx.com rbd-target-api[20832]: rv = self.dispatch_request()
Aug 24 03:40:02 node2.ceph.xxxx.com rbd-target-api[20832]: File "/usr/lib/python2.7/site-packages/flask/app.py", line 1461, in dispatch_request
Aug 24 03:40:02 node2.ceph.xxxx.com rbd-target-api[20832]: return self.view_functions[rule.endpoint](**req.view_args)
Aug 24 03:40:02 node2.ceph.xxxx.com rbd-target-api[20832]: File "/usr/bin/rbd-target-api", line 94, in decorated
Aug 24 03:40:02 node2.ceph.xxxx.com rbd-target-api[20832]: return f(*args, **kwargs)
Aug 24 03:40:02 node2.ceph.xxxx.com rbd-target-api[20832]: File "/usr/bin/rbd-target-api", line 475, in _gateway
Aug 24 03:40:02 node2.ceph.xxxx.com rbd-target-api[20832]: gateway.manage(target_mode)
Aug 24 03:40:02 node2.ceph.xxxx.com rbd-target-api[20832]: File "/usr/lib/python2.7/site-packages/ceph_iscsi_config/gateway.py", line 420, in manage
Aug 24 03:40:02 node2.ceph.xxxx.com rbd-target-api[20832]: self.load_config()
Aug 24 03:40:02 node2.ceph.xxxx.com rbd-target-api[20832]: File "/usr/lib/python2.7/site-packages/ceph_iscsi_config/gateway.py", line 256, in load_config
Aug 24 03:40:02 node2.ceph.xxxx.com rbd-target-api[20832]: lio_root = root.RTSRoot()
Aug 24 03:40:02 node2.ceph.xxxx.com rbd-target-api[20832]: File "/usr/lib/python2.7/site-packages/rtslib_fb/root.py", line 85, in __init__
Aug 24 03:40:02 node2.ceph.xxxx.com rbd-target-api[20832]: self._set_dbroot_if_needed()
Aug 24 03:40:02 node2.ceph.xxxx.com rbd-target-api[20832]: File "/usr/lib/python2.7/site-packages/rtslib_fb/root.py", line 167, in _set_dbroot_if_needed
Aug 24 03:40:02 node2.ceph.xxxx.com rbd-target-api[20832]: fwrite(dbroot_path, self._preferred_dbroot+"\n")
Aug 24 03:40:02 node2.ceph.xxxx.com rbd-target-api[20832]: File "/usr/lib/python2.7/site-packages/rtslib_fb/utils.py", line 79, in fwrite
Aug 24 03:40:02 node2.ceph.xxxx.com rbd-target-api[20832]: file_fd.write(str(string))
Aug 24 03:40:02 node2.ceph.xxxx.com rbd-target-api[20832]: IOError: [Errno 22] Invalid argument
Aug 24 03:40:02 node2.ceph.xxxx.com rbd-target-api[20832]: Change detected - internal 2 / xattr 3 refreshing
```