[v2] neutron: allow omission of subnet_id for IP address

[gophercloud-backport-bot]

Backport: #3448

I could not find any docs about external_fixed_ip struct to support omitted subnet_id, but I created test case in terraform provider and it works as expected, see terraform-provider-openstack/terraform-provider-openstack#1915

As for the port's subnet_id, the omission behavior is documented here:

The IP addresses for the port. If you would like to assign multiple IP addresses for the port, specify multiple entries in this field. Each entry consists of IP address (ip_address) and the subnet ID from which the IP address is assigned (subnet_id).
If you specify both a subnet ID and an IP address, OpenStack Networking tries to allocate the IP address on that subnet to the port.
If you specify only a subnet ID, OpenStack Networking allocates an available IP from that subnet to the port.
If you specify only an IP address, OpenStack Networking tries to allocate the IP address if the address is a valid IP for any of the subnets on the specified network.

[coveralls]

coverage: 63.571%. remained the same
when pulling a776073 on bp-v2-3a9dda3
into 01ba8ed on v2.

[kayrus]

tests fail because of the https://review.opendev.org/c/openstack/neutron/+/951136