Support ovn_underlay_*subnet* in preseed

In the following example in the test case, there is an unnecessary conversion from a subnet 10.3.123.0/24 to the actual IP addresses such as 10.3.123.2, .3, and .4 before passing the value to the preseed file.

https://github.com/canonical/microcloud/blob/9df6e15e0f7227f1d117a9d5d422cfb6ac5dcfff/test/suites/preseed.sh#L30-L36

It would be nice if the preseed can simply accept a subnet instead of an explicit IP address.

[actual]
```yaml
systems:
- name: micro01
ovn_uplink_interface: enp6s0
ovn_underlay_ip: 10.3.123.2
- name: micro02
ovn_uplink_interface: enp6s0
ovn_underlay_ip: 10.3.123.3
```

[expected]
```yaml
systems:
- name: micro01
ovn_uplink_interface: enp6s0
ovn_underlay_subnet: 10.3.123.0/24
- name: micro02
ovn_uplink_interface: enp6s0
ovn_underlay_subnet: 10.3.123.0/24
```

In a similar way with `lookup_subnet`, `ceph.internal_network`, and `ceph.public_network`.

Start with test/suites/preseed.sh around lines 30-36 and trace how the preseed fields are handled. Update the preseed behavior and related tests so ovn_underlay_subnet accepts 10.3.123.0/24, with analogous support for lookup_subnet, ceph.internal_network, and ceph.public_network; done means the explicit IP conversion is no longer needed.

Written by the indexing model from the issue text.