ovs-vsctl: additional commands for querying VLAN fake bridges.

ovs-vsctl: additional commands for querying VLAN fake bridges.

Our test automation needs to be able to validate that a VLAN bridge and
for this I needed two new operations in ovs-vsctl:
      * The ability to query the VLAN tag for a bridge.
      * The ability to query the 'parent' of a bridge. The parent is the
        non-VLAN/untagged bridge with the same physical devices and
        could be a bond.

So given xenbr0 (containing eth0) + xapi2 (VLAN 42 on eth0) and xapi1
(containing bond0 == eth2+eth3) + xapi3 (VLAN 23 on the bonded
interface):

[root@warlock ~]# ovs-vsctl br-to-vlan xapi2
42
[root@warlock ~]# ovs-vsctl br-to-vlan xapi3
23
[root@warlock ~]# ovs-vsctl br-to-parent xapi2
xenbr0
[root@warlock ~]# ovs-vsctl br-to-parent xapi3
xapi1