Tag the alembic migration revisions for Liberty

Tag the alembic migration revisions for Liberty

Previously when we had one repo with one alembic branch we would
create a milestone revision on that single branch. Now we have
multiple repos and expand/contract branches for each repo.

So from now on we tag the final revision on every branch when we make
a milestone release. Update the cli to support the command:
  neutron-db-manage upgrade <milestone>
where <milestone> becomes an alias for all the revisions for a
milestone.

Partial-Bug: #1499033

Change-Id: I38623986dd574bec01fe147f9c6a747f3f512bb7