Fix underlay configuration for preseed.yaml

[masnax]

The logic for detecting the OVN underlay configuration for preseed was not correct, it was relying on c.state which contains the resources available and already set up on existing cluster members, as well as new cluster members. It is used in interactive setup to manage additional configuration being supplied to the existing systems.

In preseed, we don't populate it yet because we only want to apply the precise recipe that is defined in the preseed, and not change any other cluster information implicitly (in interactive setup we can ask the user).

Additionally, the input parsing was not correct, since it expected a CIDR per system however the ovn encap configuration wants an IP.

So this addresses that by instead using the list of systems parsed from the yaml itself in p.Systems, and uses the same set of interfaces that were detected for the internal ceph cluster network, since they have the same restrictions. Also, rather than specifying a CIDR per system, instead we can specify the encap IP directly, and validate that it fits in one of the available subnets.

Preseed tests have also been updated for the new key, which is prefixed with ovn_ now to be clear what it affects. The cephfs key has also been moved into the global ceph configuration because it's not system-specific.

[roosterfish]

LGTM! Only some minor comments but they can also be addressed later.

[roosterfish]

LGTM, there is only one warning about usage of Println instead of Printf.

[gabrielmougard]

It looks good!

[roosterfish]

Only two smaller comments, rest still looks good.

[masnax]

Looks like SupportsFeature still doesn't work because we haven't added the fix to a tagged release yet.