Dropped release name from migration branch labels

Dropped release name from migration branch labels

Since the plan is to attach first Mitaka scripts to Liberty branches
with down_revision, and since labels are inherited from all other
revisions in the chain, using release names in branch labels would mean
that the following commands would be valid:

neutron-db-manage upgrade liberty_expand@<some_revision_from_mitaka>
neutron-db-manage upgrade mitaka_expand@<some_revision_from_liberty>

which may be confusing to users.

So let's drop release names from branch labels and use expand@head and
contract@head to access latest migrations.

Change-Id: Id524d7673ad248c831f6dbb3a6f2f3c50094acae
Partially-Implements: blueprint online-schema-migrations