webadmin: disks subtab - enable template disk removal

Enable template disk removal in disks sub-tab
(under storage main-tab).

Change-Id: Ieff2f7a999cc1baa34ae85c34a51c83e19600bed
Bug-Url: https://bugzilla.redhat.com/892714
Signed-off-by: Daniel Erez <derez@redhat.com>