Add backend for GlusterFS, Nexanta, SAN, and SolidFire

Some drivers could not be used in multi backends mode. This
commit adds a backend class for theses drivers and make use
of them in their respective cloud::volume::* class

Change-Id: I2939610cfc078d6ec6b85e5516138620b106237f
Merge "Add backend for GlusterFS, Nexanta, SAN, and SolidFire"