Fixes bug 1046952
In create/update_subnet(), IP address fields (cidr, gateway_ip, host_routes,
dns_nameserver) should be checked whether they are consistent with IP version
specified. IP version check for allocation_pools is already implemented,
so this commit adds IP version checks for other IP fields.
Change-Id: Ic02f5a1fd8ec4c1c363599c906ba0c818dd22268