Those classes handles ML2 plugin's configuration related to the
OVS and SR-IOV mechanism drivers.
Previously there was neutron::plugins::ml2::mech_driver function which
was used by neutron::plugins::ml2 class but it isn't used at all
currently and probably it will need to be deprecated and removed in
follow up patch(es).
Change-Id: I2a321fb780e789b7556d557e2170a3267049c432