Apply six for metaclass

__metaclass__ cannot be used in python3.
six be used in general for python 3 compatibility.

Change-Id: I0d7c61b4c0103d4f9e627ff5d22cddcdff2f7274
Add datastore version to backups

This fix adds datastore information to the backup APIs.
These API includes backup list, backup list by instance and
backup show.

Also this fix enhances backup-list to optionally receive a
datastore to filter the backups by.

The integration tests will fail until the update for the client
gets merged: https://review.openstack.org/#/c/90462/

Change-Id: Icfec975b92cd9523e639ad6a2d6787ee4d4cb39d
Implements: blueprint backup-metadata
Imported Translations from Transifex

Change-Id: I01cc796de1f7fd9b6da3e45fb172990a623ec27d
Merge "Imported Translations from Transifex"
Merge "Apply six for metaclass"