neutron-db-manage: sync HEADS file with 'current' output

neutron-db-manage: sync HEADS file with 'current' output

alembic.get_heads() returns all heads for all branches it can find in
scripts dir, while in alembic_version table, it does not store any heads
that were overridden by other branches, even if those depends_on it
instead of having it as down_revision.

To keep 'current' output in sync with what is in HEADS file, we can
attach liberty_* branches explicitly to kilo revision.

It's also a good idea to have a separate 'heads' command that would show
the latest alembic heads based on scripts dir state. See [1] for more
details.

While at it, since different subprojects can link their expand/contract
branches to kilo in different way (some using depends_on the previous
release branch, while others, as suggested in this patch, thru
down_revision to kilo), we kill the check on the number of heads
returned by script.get_heads() since it may differ. If we want to
validate that we don't branch more than twice from kilo, we may add a
separate validation just for that.

[1]: https://review.openstack.org/#/c/204551/

Change-Id: If551633ab26e0eac549c1e13cfa0771383a1a060
Partially-Implements: blueprint online-schema-migrations