Include alembic migrations in module

Explicitly include all alembic data in installed python module,
ensuring that installations direct from git repositories are
fully functional when compared to use of sdist generated tarballs
from tarballs.openstack.org or pypi.

Change-Id: Ica6e8e84f5bb1638ad3e7042fac381aef2c14edf
Closes-Bug: 1574610

MANIFEST.in

@@ -3,4 +3,8 @@ include ChangeLog
 exclude .gitignore
 exclude .gitreview
 
+include networking_l2gw/db/migration/alembic_migrations/README
+include networking_l2gw/db/migration/alembic_migrations/script.py.mako
+recursive-include networking_l2gw/db/migration/alembic_migrations/versions *
+
 global-exclude *.pyc