[r=mpontillo][bug=1699479][author=danilo] Make TestDHCPNetworkLayout.test__dhcp_configurations_rendered always run in both IPv4 and IPv6 network configurations to ensure it stably either passes or fails.
To avoid resolve_hostname() on the MAAS configured URL only return an IP address that it resolves to on the local system (which could be only IPv4 or only IPv6 address) which can't be used for a DNS server, we patch resolve_hostname() in the test to return the appropriate localhost address.