Fix assert for public networks with multiple subnets
Fix assert for public networks with multiple subnets

_verify_gateway_port() currently assumes that the fixed ip
for the router gateway is allocated from the first subnet of
the public network. This is not always the case if there
are multiple subnets on the public network.

Change-Id: Ib13ba4aaed42f62397c7db165e765e47d4d49232
Closes-Bug: 1624057

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