Validate that there's no duplicate volume name in proposal
Make boot.looplvm support multiple loop devices
Validate that nodes have enough disks for all raw backends

This doesn't take into account the fact that some backends might request
all disks, but that's good enough for now.
Only allow selecting one disk when using multiple raw devices
Avoid duplicate chef resources for local/raw backends

Also make logs a bit more explicit
Validate that local backends don't use the same file name