Cisco Neutron plugin fails DB migration

Alembic migration from revision f44ab9871cd6 to 2eeaf963a447 fails
because the floatingips table doesn't exist.

The fix is to add the plugin to the DB migration path for l3 and ext_gw_mode.

Change-Id: Ic7a9c1d624d2838e9313fa95ed6ddbfea9b6e1c9
Closes-Bug: #1289039
Merge "Cisco Neutron plugin fails DB migration"