homed: default to "btrfs" as fs type in the LUKS backend

Apparently both Fedora and suse default to btrfs now, it should hence be
good enough for us too.

This enables a bunch of really nice things for us, most importanly we
can resize home directories freely (i.e. both grow *and* shrink) while
online. It also allows us to add nice subvolume based home directory
snapshotting later on.

Also, whenever we mention the three supported types, alaways mention
them in alphabetical order, which is also our new order of preference.