Adds the external_resource_vip_id property, which can be used to set an
external_id for the port resource for the network VIP.
Since the same template resource, port.network.j2.yaml is used for both
VIP and normal ports on a network, we can't simply add jinja to that
template that conditionally adds the external_id attribute because we
don't know during the jinja2 phase if the template is for a VIP or not.
Instead, we need to map the VIP resources to an entirely new template
resource (external_resource_port.network.j2.yaml) so that we can set the
external_id attribute just for the VIP ports.
implements: blueprint split-controlplane-templates