Mock cinder.wsgi.Server in TestWSGIService

The fixed tests were binding to port 8776. These unit tests should not
be starting the wsgi app and so the class has been mocked.

Change-Id: I52b8ce4788741a0802d4d91d413474101542a3e0
Closes-bug: #1392498
Merge "Mock cinder.wsgi.Server in TestWSGIService"