Opscenter is not starting correctly on latest builds

The following script:

```
ccm create -n 3 --dse --dse-username= --dse-password= -v 4.8.8 -o 5.0.2 test
ccm start
```

results in the following error:

```
Traceback (most recent call last):
File "/home/datastax/.local/bin/ccm", line 86, in
cmd.run()
File "/home/datastax/.local/lib/python2.7/site-packages/ccmlib/cmds/cluster_cmds.py", line 583, in run
allow_root=self.options.allow_root) is None:
File "/home/datastax/.local/lib/python2.7/site-packages/ccmlib/dse_cluster.py", line 68, in start
self.start_opscenter()
File "/home/datastax/.local/lib/python2.7/site-packages/ccmlib/dse_cluster.py", line 90, in start_opscenter
self.write_opscenter_cluster_config()
File "/home/datastax/.local/lib/python2.7/site-packages/ccmlib/dse_cluster.py", line 113, in write_opscenter_cluster_config
(seed_ip, seed_port) = seed.network_interfaces['thrift']
AttributeError: 'str' object has no attribute 'network_interfaces'
```

This is because when a cluster is reloaded the seeds are loaded as a list of strings rather than nodes.