webadmin: Add no-quota notification and quota buttons

Sub-tab quota under the main-tab Data Center had no action buttons (to
add, edit, copy or remove quota).

When enabling quota in a Data Center, we'd like to notify the user he
needs to define quotas. Otherwise system users may be blocked from
running VMs or creating new disks.

This patch will add such a notification when the data center has no
quotas defined.

DataCenterQuotaListModel was changed to inherit QuotaListModel in order
to include all CRUD actions.

Change-Id: Ie5d360af1329df9f274f9e9651e3c0d9a8053f1b
Bug-Url: https://bugzilla.redhat.com/845022
Bug-Url: https://bugzilla.redhat.com/855630
Signed-off-by: Ofri Masad <omasad@redhat.com>