Accept deleting multiple snapshots in one shot

In bug 1241941 the delete and force-delete commands were updated to
accept multiple volumes for deleting in the same call. This patch adds
the same behaviour to the snapshot-delete command.

Closes-Bug: #1284540
Change-Id: I5e4bf8641d7fd261fa24b03832bad08007e43c12
Merge "Accept deleting multiple snapshots in one shot"