Added '' as a valid value for gateway_ip

If the gateway_ip attribute on neutron_subnet is set to an empty string
('') then it will trigger removing the DHCP gateway setting from the
subnet.  This is equivalent to the --no-gateway flag in the CLI.

Change-Id: Id4b74062d5b5b2fb335167af4e7cfcc32c4db703
(cherry picked from commit d202e4bbf70be6d7e70a87e5be334c0b69f3516c)
Merge "Added '' as a valid value for gateway_ip" into stable/icehouse