The ``neutron-ovn-db-sync-util`` replaces the ``ovn`` mechanism
driver with the ``ovn-sync`` mechanism driver. Subsequently it
makes calls into the OVN L3 service plugin.
At present the OVN L3 service plugin assumes the ``ovn`` mechanism
driver to be present and produces a Traceback when it is not.
This patch fixes that by testing for both of the supported
mechanism drivers.
Change-Id: I1ac685a12f49119f5ef1428cbc504b639d783803
Closes-Bug: #1882202
Signed-off-by: Frode Nordahl <frode.nordahl@canonical.com>