Fixed erroneous force full copy in ceph backup driver

The following steps should not result in a forced full copy:

1. create vol
2. create backup (works fine i.e. uses incremental diff)
3. delete backup
4. create new backup (full copy forced because volume backup-snap exists and is not ignored)

This patch resolves the above problem.

Change-Id: I61c245219f54f7ee942e06e343c5d79d4cab947b
Fixes: bug #1221836
Merge "Fixed erroneous force full copy in ceph backup driver"