HEAD file for alembic migration

Add the new HEAD file for alembic migration to keep track of the
latest revision.

Change-Id: I1f6e8dbc85033ef676e08494da78d3a36cabe5e6
Missing __init__.py files

Add all the missing __init__.py files under neutron.db.migration

Change-Id: I4c3b55f539445a866fa35b4f895bb1193c622e3b