Fix Pure get pgroup volume snapshot name

Fix Pure get pgroup volume snapshot name

When creating a volume from a consistency group snapshot we have to go
look up the volume level snapshot from the group. Previously we could
just directly get to the cgsnapshot model from the snapshot. After we
switched to using the snapshot objects however we cannot do that (at
least not until we have cgsnapshot objects too).

As an interim solution we did a query to the REST API on the array to
just list all of the snapshots. Unfortunately that doesn’t work on all
versions of the Purity REST API. To have a more usable solution we need
to do a db lookup to get the pgsnapshot.

It’s not ideal, and will be removed ASAP, but for now we need to at
least not regress in functionality.

Change-Id: I73a4ad5883bf33ce282b5a49aa8acba66ffcb84f
Closes-Bug: #1494934