webadmin: Storage Snapshots sub-tab

Introducing new sub-tab: Storage -> Snapshots
(includes all disks snapshots that reside on
the selected storage domain).

Change-Id: I888dad6a3f7ac876a859ce5e8e371599d0873b22
Feature-Page: http://www.ovirt.org/Features/Snapshots_Overview
Signed-off-by: Daniel Erez <derez@redhat.com>