Subnet sharing warning should check interfaces not IPs

[roosterfish]

The warning in https://github.com/canonical/microcloud/blob/main/cmd/microcloud/main_init.go#L530 isn't fired in case OVN underlay configuration is using an IPv4 and Ceph public/internal traffic is using an IPv6.

If both IP networks are configured on the same interface the warning should still be fired to notify the user that a single interface is used for multiple networks that maybe should have been segregated.

Starting from #515 bridges can be used for both the OVN uplink and network segregation so checking the interfaces too would make it more consistent.