Add snapshots to the volume usage audit report

Add snapshots to the volume usage audit report

* Added tests for db api call volume_get_active_by_window called by the volume
usage audit report.

* Added new db api called snapshot_get_active_by_window & test it. This is
similar to the volume_get_active_by_window method but returns the snapshots
 active the supplied time window.

* Integrate the above into the cinder-volume-usage-audit script to report on
  all the active snapshots as well as volumes in the specified time frame.

Fixes: bug #1157110
Change-Id: I7597da20c62150a8f6c74cbfeec159f68064d2a6