Cinder backends

For both HA and non-HA controllers:

* allow multiple backends

* Dell EqualLogic backend

For non-HA controller:

* NFS backend (as an effect of using the same base classes as HA controller)
I think not, because it's nfs client, not nfs server (same as with Gluster). Afaik the only backend that really hosts the storage is iscsi, others just do mounting.