Include alembic migrations in module

When packaging networking-vpp using a different mechanism than py
sdist tarballs, files that need to reside in the python module are
missed, leading to silent errors - the package contains everything
needed for normal operation, but trying to apply alembic migrations
will report success without changing anything.

This used to be a common issue with various Openstack projects, e.g.
see [1].

[1] https://bugs.launchpad.net/ubuntu/+source/networking-odl/+bug/1627656

Change-Id: I3b8dc26d987fea04a8dd11a3a9d4110a1aa3eac8
Signed-off-by: Alexandru Avadanii <Alexandru.Avadanii@enea.com>

MANIFEST.in

@@ -0,0 +1,7 @@
+include networking_vpp/db/migration/alembic_migrations/script.py.mako
+recursive-include networking_vpp/db/migration/alembic_migrations/versions *
+
+exclude .gitignore
+exclude .gitreview
+
+global-exclude *.pyc