engine: sorting gluster volume options

Sorting the gluster volume options returned from vdsm
based on their keys.
Replaced HashSet with TreeSet to preserve the ordering.

Change-Id: Iea091e1d06fefb9ef4710ab68896614fcc01bb9f
Bug-Url: https://bugzilla.redhat.com/973635
Signed-off-by: Kanagaraj M <kmayilsa@redhat.com>