Cinder multi-backend and Dell EqualLogic support

Cinder multi-backend and Dell EqualLogic support

It is dependent on: https://review.openstack.org/89602

This is a superset of https://github.com/redhat-openstack/astapor/pull/184

Bugzilla Ref:
https://bugzilla.redhat.com/show_bug.cgi?id=1094385