To allow MechanismDrivers to start their own RPC listeners (e.g. for
communication with custom agents) the MechanismManager will now call
start_rpc_listeners() of each driver. This is done as part of
Ml2Plugin.start_rpc_listeners(). It is added as an alternative to create
the backends in initialize(), as in cases where a driver is split up
into the API and RPC part we want to make sure these backends are only
started in the RPC part of neutron.

This patch depends on MechanismDrivers.start_rpc_listeners() in
neutron-lib[0].

[0] https://review.opendev.org/c/openstack/neutron-lib/ /919589

Change-Id: I31e253180f474abf6d266d23c50f9dc89f17f687
Depends-On: https://review.opendev.org/c/openstack/neutron-lib/ /919589
Closes-Bug: #2065198

We do not need to call start_rpc_workers() to start the rpc listeners of
our ml2 drivers, as these are now started via start_rpc_listeners(),
which is called from within Ml2Plugin's own start_rpc_listeners()
method.

All drivers that rely on their listeners to be started via get_workers()
and do not have a proper start_rpc_listeners() method will need to
implement one, but as most use get_workers() to just call their own
start_rpc_listeners() there shouldn't be much to be done on the driver's
side.

This reverts commit b2376f8.