extend subnet options

Description

We need ability to create openstack network without dhcp, and change gateway-ip

required by: kolla-virtualized-infrastructture

benefit: our virtualize infrastructure will be more similar to physical setup

Proposal

options to add to setting subnet

gateway_ip: "{{ _openstack_network.subnet_gateway | default(omit) }}"
enable_dhcp: "{{ _openstack_network.subnet_enable_dhcp | default(omit) }}"

Links / references