glusterfs: Fix issues in backend instrumentation

- allow share or sub-directory access to more than one guest/ip address
  by modifying the GlusterFS volume configuration options that change
  the Gluster-NFS server settings.

  `nfs.export-dir` option does not interpret a spec of
  '/dir(host1),/dir(host2)'as we expected, ie. two independent access
  specifiers -- instead, it just discards the latter. The required
  format looks as follows: '/dir(host1|host2)'. Update the export-dir
  generating routines accordingly.

- properly remove the last share or sub-directory access setting in a
  GlusterFS volume when needed by an API request to changes access
  rules of shares.

  `volume set` option in GlusterFS volume settings cannot take empty
  value, use instead `volume reset` to go back to defaults

- GlusterFS volume contains all the shares. It has a default setting
  that allows NFS access to its entire contents. So turn off this
  setting as a part of the initial driver setup.

  turn off `nfs.export-volumes` setting.

Change-Id: I652a13d4d34d1b4c5ee3d31af40143a098c38074