Made provision for multi-store support. Added new config option
'enabled_backends' which will be a comma separated Key:Value pair
of store identifier and store type.
DocImpact
Depends-On: https://review.openstack.org/573648
Implements: blueprint multi-store
Change-Id: I9cfa066bdce51619a78ce86a8b1f1f8d05e5bfb6