OpenStackLatentWorker does not interpolate parameters

Buildbot version: 2.5.0

With following snippet
```.py
c['workers'].append(worker.OpenStackLatentWorker('openstack-worker-01', 'slavepass',
image = util.Interpolate('%(prop:os_image_uuid)s'),
flavor = util.Interpolate('%(prop:os_image_flavor)s'),
missing_timeout = 600
))
```

And with scheduler

```.py
c['schedulers'].append(schedulers.ForceScheduler(
name="test-ci",
buttonName="Run",
label="Run test CI",
builderNames=["test-ci"],
properties=[
util.FixedParameter(name="os_image_uuid", default="b9c6f993-6b4a-44a0-accf-c5c3e1536a88"),
util.FixedParameter(name="os_image_flavor", default="c931aa9d-7c81-4dfe-9c2a-4912b161ba1d")
],
codebases=[util.CodebaseParameter(codebase='', repository='', revision='', branch='', hide=True)],
reason=util.FixedParameter(name="reason", default="Forced build"),
username=util.FixedParameter(name="username", default="user"),
))
```

We get following error in buildbot

```
Traceback (most recent call last):
File "/opt/buildbot/lib/python3.5/site-packages/twisted/internet/defer.py", line 654, in _runCallbacks
current.result = callback(current.result, *args, **kw)
File "/opt/buildbot/lib/python3.5/site-packages/twisted/internet/defer.py", line 1475, in gotResult
_inlineCallbacks(r, g, status)
File "/opt/buildbot/lib/python3.5/site-packages/twisted/internet/defer.py", line 1416, in _inlineCallbacks
result = result.throwExceptionIntoGenerator(g)
File "/opt/buildbot/lib/python3.5/site-packages/twisted/python/failure.py", line 491, in throwExceptionIntoGenerator
return g.throw(self.type, self.value, self.tb)
--- ---
File "/opt/buildbot/lib/python3.5/site-packages/buildbot/process/build.py", line 346, in startBuild
ready_or_failure = yield workerforbuilder.prepare(self)
File "/opt/buildbot/lib/python3.5/site-packages/buildbot/worker/latent.py", line 265, in _substantiate
start_success = yield self.start_instance(build)
File "/opt/buildbot/lib/python3.5/site-packages/twisted/internet/defer.py", line 1416, in _inlineCallbacks
result = result.throwExceptionIntoGenerator(g)
File "/opt/buildbot/lib/python3.5/site-packages/twisted/python/failure.py", line 491, in throwExceptionIntoGenerator
return g.throw(self.type, self.value, self.tb)
File "/opt/buildbot/lib/python3.5/site-packages/buildbot/worker/openstack.py", line 218, in start_instance
block_devices)
File "/opt/buildbot/lib/python3.5/site-packages/twisted/python/threadpool.py", line 250, in inContext
result = inContext.theWork()
File "/opt/buildbot/lib/python3.5/site-packages/twisted/python/threadpool.py", line 266, in
inContext.theWork = lambda: context.call(ctx, func, *args, **kw)
File "/opt/buildbot/lib/python3.5/site-packages/twisted/python/context.py", line 122, in callWithContext
return self.currentContext().callWithContext(ctx, func, *args, **kw)
File "/opt/buildbot/lib/python3.5/site-packages/twisted/python/context.py", line 85, in callWithContext
return func(*args,**kw)
File "/opt/buildbot/lib/python3.5/site-packages/buildbot/worker/openstack.py", line 227, in _start_instance
instance = self.novaclient.servers.create(*boot_args, **boot_kwargs)
File "/opt/buildbot/lib/python3.5/site-packages/novaclient/v2/servers.py", line 1434, in create
return self._boot(response_key, *boot_args, **boot_kwargs)
File "/opt/buildbot/lib/python3.5/site-packages/novaclient/v2/servers.py", line 828, in _boot
return_raw=return_raw, **kwargs)
File "/opt/buildbot/lib/python3.5/site-packages/novaclient/base.py", line 364, in _create
resp, body = self.api.client.post(url, body=body)
File "/opt/buildbot/lib/python3.5/site-packages/keystoneauth1/adapter.py", line 381, in post
return self.request(url, 'POST', **kwargs)
File "/opt/buildbot/lib/python3.5/site-packages/novaclient/client.py", line 78, in request
raise exceptions.from_response(resp, body, url, method)
novaclient.exceptions.BadRequest: Flavor Interpolate('%(prop:os_image_flavor)s') could not be found. (HTTP 400) (Request-ID: req-4eb9d628-e886-45ae-b9f2-2d0d62dbb0ab)
```

So the flavor parameter, ~~and probably image too~~ (update: image works just fine), are not being actually interpolated but somehow force-mangled into strings causing really unexpected behaviour.

Also the HTTP 400 error causes RETRY when it should probably not.