glusterfs: add infrastructure to accommodate NAS helpers

- Refactor the main driver class, GlusterFSShareDriver, to make it
  pluggable with different NAS helpers.

   - Add GlusterNFS helper class to manage shares served by Gluster-NFS
     server. The management of these share was earlier done within the
     main driver class.

- Enhance the methods of the GlusterAddress class that would be used
  by the main driver class and the helper classes. Rename the
  GlusterAddress class to GlusterManager class. This class would
  contain the methods to interface with the backend GlusterFS volumes.

   - Retire GlusterAddress.make_gluster_args() in favor of
     GlusterManager.make_gluster_call(). The make_gluster_call() method
     implements a more correct approach to remote execution. It's
     remote execution is based on processutils.ssh_execute. The
     make_gluster_args() method that it replaces facilitated remote
     execution by prefixing arguments with 'ssh'.

   - Move the interface used to fetch the value of an option set on a
     GlusterFS volume, from the main driver class to the GlusterManager
     class.

Partially implements blueprint gateway-mediated-with-ganesha

Change-Id: I3cbeb49c26f5f24152b42b649ce3bc75451964ef