cli: output xml in pretty format

Gluster cli now prints XML outputs in 'pretty' format. This solves the problem
of empty elements occuring as two tags instead of being collapsed into one.

Change-Id: Iab7aeadcff29c18ae388b58e446a16e937ac09ed
BUG: 849293
Signed-off-by: Kaushal M <kaushal@redhat.com>
Reviewed-on: http://review.gluster.org/4355
Reviewed-by: Krutika Dhananjay <kdhananj@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Anand Avati <avati@redhat.com>