add HEAD sentinel file that contains migration revision

add HEAD sentinel file that contains migration revision

This change wraps the command to generate a migration with code to
update a file called HEAD. The HEAD file will contain the revision ID
of the head of the migraton timeline. Additionally, check migrations has
been altered to verify the contents of this file against the timeline
head.

This file helps the OpenStack gate detect potential migration branches
without running alembic via git merge conflicts.

Closes-Bug:1288427
Change-Id: If382c57baea061753d3f4fcd6faec1a31fbfb7ed