3PAR volumes created from snaps failed to attach

An error would occur if you attempted to attach a volume that
was created from a snapshot. The 3PAR backends does not have a
'userCPG' entry in the volume data on the backend, it instead
has a 'snapCPG' field. This patch will now look for the 'snapCPG'
entry and use that CPG when attaching the volume created from a
snapshot.

Change-Id: Ic7dab730a68339edb4273e4fbb6f8e45e37808b3
Fixes: bug 1207913