This patch adds the ``Cluster`` and ``Backend State`` columns to
the ``openstack volume service list`` command. Note that you need
to provide the appropriate microversion to show these columns.
Cluster: openstack --os-volume-api-version 3.7 volume service list
Backend State: openstack --os-volume-api-version 3.49 volume service list
Change-Id: Ie7727d0001307b5d5a40d7ea0348bdb9626f9e03