Adding volume size to the backup views/models.

* Save the size of the guest before running a backup
* Display the size in the backup GET/LIST views
* Simplify the dbaas module removing redundant class.
* Switch get_filesystem_volume_stats to use os.statvfs()
* Change output of volume stats to return used in GB
  to simplify various view that display it.

Implements: Blueprint backup-volume-size

Change-Id: I860427d3491acb34e43ee23f91e4ffbb39e0042c
Merge "Adding volume size to the backup views/models."