Enable LUKS encryption on RAID

[Johan Ehnberg]

There is currently no alternative that replaces "physical volume for encryption" as was present in d-i.

[Michael Hudson-Doyle (mwhudson)]

We currently only support encrypted LVM. So you can create a VG containing your raid and encrypt that. Is there some reason this doesn't work for you?

[Johan Ehnberg (johan-ehnberg)]

In our case, we simply do not need LVM, so using that would be an artificial workaround to get encryption with this installer.

It could also be argued from a UX perspective that it is not intuitive to look for encryption as a feature of LVM. Doing so perhaps unnecessarily also loses the flexibility of abstraction provided by d-i.

[Johan Ehnberg (johan-ehnberg)]

As a workaround, the autoinstaller supports skipping LVM by using curtin actions directly.

Two caveats though:
1) subiquity requires byte-sizes in the config when using raid, see #1876605
2) spare_devices is required by subiquity although not by curtin, so just add 'spare_devices: []'