Added test for router's port update with fixed IP.
Added test for router's port update with fixed IP.

Test Added:
- test_router_port_update_with_fixed_ip
  This test verifies that router's port can be updated with
  new fixed ip.

Change-Id: I1d685c7ef9cd6f55958edbd25fd768cc2be2f3dc
Related-Bug: #1465434

Tesora/tesora-tempest,cisco-openstack/tempest,cisco-openstack/tempest,bigswitch/tempest,openstack/tempest,Juniper/tempest,bigswitch/tempest,vedujoshi/tempest,masayukig/tempest,openstack/tempest,Tesora/tesora-tempest,vedujoshi/tempest,Juniper/tempest,sebrandon1/tempest,masayukig/tempest,sebrandon1/tempest