feat: RAID creation: Automatically find and create next RAID device

[Hurricos]

Note that the current default actually has a bug where, despite the placeholder saying '/dev/md0', the actual path created by default is /dev/md1.

This change moves to match the user's expectations by actually using the placeholder that they see.

[Hurricos]

Force-pushed to correct the authorship of the commit.

I could probably improve the commit message to mention that this actually does a search to find the next available RAID device, but the original code did already do that, it just didn't reflect it in the placeholder.

[Hurricos]

Oh, and I could probably change the commit message to "actually using the same logic to generate a placeholder as used to select the default (since the placeholder value is not communicated)."

[lisaac]

Thanks