Add a test test_router_with_existing_ports, enhance existing router case to check for gateway IP on port

Change-Id: Ibdd879e0907b7b23f669701e4c953e8b1dada110
pep compliance for base.py and test_routers.py

Change-Id: Ia9caff1fa5a93dcc6c1a0b230cc72405cbfa3aed