Delete consistency group failed

Delete consistency group failed

Delete CG failed because it compares host@backend with host@backend#pool
(in delete_consistencygroup in volume/manager.py). The fix is to call
extract_host before doing the comparison.

Another issue is about deleting a CG with no host. This will throw exception
when extract_host(group['host']) is called in delete_consistencygroup in
volume/rpcapi.py. The solution is to check the host field in consistencygroup/
api.py and delete it from db there.

Change-Id: Ife28a4f91bd8a4e123c74dac9748ee31cc7b7306
Closes-Bug: #1366205