Add support for existing LUKS partitions

[py-crash]

I have a disk with existing luks lvm partition. I can open the existing partition using:

sudo cryptsetup luksOpen /dev/sdXY <Name>

However in the installer in "Advance disk configuration" will only see the physical partitions.

There is a workaroud for installing solus, however the LVM partitions should be listed in the installer. Ubiquity and Calamares installer already support this feature.