Return dict for external_gateway_info in router as spec'd by API.

When unset, return None for external_gw_info as expected by
tempest.api.network.test_routers.RoutersTest.test_update_router_unset_gateway

Change-Id: I89834bffec0a64ff62607dceed654b8596623b83
Closes-Bug: #1386471
(cherry picked from commit d1d062a0087270674b6557c1f8ea419d89bafdf1)