webadmin: fix tree view items sorting

Fixed items in the left tree being not sorted under each DC.
The fix applies to: storage domains, networks and clusters.

Change-Id: Icea17fd037ea4c97cc9a9976eda869b73e489f21
Bug-Url: https://bugzilla.redhat.com/1045380
Signed-off-by: Alissa Bonas <abonas@redhat.com>