gluster: new vdsm verb glusterHostUUIDGet

New gluster vdsm verb to get glusterd UUID of the host, which uses
gluster cli command "gluster system:: uuid get"

Gluster CLI sample output:
UUID: f2023224-2ae2-4893-92ab-b078defd2991

new verb: glusterHostUUIDGet
Output structure: {'uuid': UUID}

Change-Id: Idf48b4c9530c48b2c46234d98c8cb4244adf22fd
Signed-off-by: Aravinda VK <avishwan@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/13785
Reviewed-by: Dan Kenigsberg <danken@redhat.com>