* Create class cinder::backend.
Sets DEFAULT/enabled_backends
* Create define cinder::type.
multip-purpose function to create type and optionally set extened
information.
* Create define cinder::type_set.
multi-purpose set function to set extended information.
* Copy rbd, iscsi, and nfs volume providers to support multi-backend
Now provided as cinder::backend::<driver> to ensure that multi-backend
aware resource is|[ not] used.
* Add rspec for new parts to ensure expected function.
Blueprint: cinder-volume-multi-backend
Change-Id: I6df9cadaad3b9ce2ab4b0e87dba9b5056459db10
209a5bd36a