Skip adding host to aggregate with az if host already in other zone

Running test_aggregate_add_host_create_server_with_az fails if all
hosts are already in other zones, with error info like:
   "conflictingRequest":
       {"message": "Cannot add host to aggregate 62.
                    Reason: One or more hosts already in availability
                    zone(s)[u'rally_test'].", "code": 409}

This is to find a host that has not been added to other zone, and
skip test_aggregate_add_host_create_server_with_az if all hosts in
the system are already in other zones.

Change-Id: I996d257e089f0676e316f2b14207c343063ccf8e
Closes-Bug: #1748377
1 file changed