Desupport neutron openvswitch and linuxbridge monolithic plugins

These plugins were removed in Juno.
Note: this doesn't affect the corresponding ML2 mechanism drivers.

Partial-Bug: #1323729
Change-Id: Ia8da1e20a03fef5657ba1584bf83ddd224b5d5f2
Merge "Desupport neutron openvswitch and linuxbridge monolithic plugins"