Fix Storwize terminate_connection with no host

Nova may pass a connector to Cinder with no 'host' field, which was
causing a KeyError in the Storwize driver. This patch resolves this case
by doing the following:
1. If the volume is mapped to only 1 host, unmap it
2. If the volume was not mapped or mapped to multiple hosts, print a
   warning but don't raise an exception

Change-Id: I0ec1c24adbdfbcf1c4868b4981a2e2618d4b411c
Closes-Bug: #1244257
(cherry picked from commit 8185d1b5db421441ebba128f23f840a42f9bf050)
Merge "Fix Storwize terminate_connection with no host" into stable/havana