Add ext-gw-mode support to ML2

Add ext-gw-mode support to ML2

Closes-Bug: #1212772

ML2 doesn't support ext-gw-mode, resulting in L3 agents not
setting-up NAT rules in neutron routers (currently, l3 agents
will assume enable_snat=false if plugin doesn't provide this
parameter as described in bug 1212868)

This patchset adds the support of ext-gw-mode to ML2 plugin.

In order to support ext-gw-mode alembic migration (as well as
other potential migrations on plugin dbs), folsom_initial
migration script is fixed to include ML2 plugin. Otherwise
databases won't be set-up during migration script (but at
plugin startup by sqlalchemy), resulting in migration failures.

Change-Id: I95ec18e0a4f0e5b661f3a5d679c434f18f100fa6