Removed ATTR_NOT_SPECIFIED values from subnet request body
Fixed create/update of Subnet.name
Implemented gateway_ip default on create_subnet
cleanup
Implemented update_subnet on gateway_ip
Fixed dns_nameservers on create_subnet
Refactored models.DNSNameserver creation into db_api.dns_create
Fixed ip data type in dns_create
Implemented dns_nameservers attribute for update_subnet