Add nec plugin to allowed address pairs migration

Change-Id: I2a6488482abacf87fc27fb3211041d81a4c1c047
Closes-Bug: #1295999
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
Merge "Add nec plugin to allowed address pairs migration"
Merge "Add script to migrate ovs or lb db to ml2 db"