Add script to migrate ovs or lb db to ml2 db

Add script to migrate ovs or lb db to ml2 db

This script migrates an ovs or lb database to work with ml2.
The script's docstring provides details as to how it is intended to
work.

I've tested this manually on a trivial deployment, and would like
to add support for testing the migration with grenade.  However,
that will have to wait until grenade support for neutron has
merged.

DocImpact

Implements: blueprint ml2-deprecated-plugin-migration

Change-Id: I1eb908c866e19f72e61f687d56a0bc391cdb4760