GlusterFS: Complete snapshot_delete when info doesn't exist

The snapshot_delete operation will fail if the snapshot info file
doesn't contain a record for the snapshot, or does not exist.
This happens in cases such as when snapshot_create fails to commit
anything to disk.

The driver should allow the manager to delete the snapshot
in this case, as there is no action required for the driver
to delete anything.

Closes-Bug: #1252864

Change-Id: I8686a1be09dbb7984072538bff6c026bb84eeb52
Remove test that was no longer used for long

test_too_big_volume() case in test_volume.py was legacy from nova-
volume, it was never actually testing anything.  This change removes
this case for good.

Change-Id: I66e0eb80326f039ddc580dec0409fd4090ba5102
Merge "Remove test that was no longer used for long"
Merge "GlusterFS: Complete snapshot_delete when info doesn't exist"