BTRFS subvolumes

BTRFS supports subvolumes (similar to zpool+zfs / lvm_volgroup+lvm_partition).

Current situation:
With
```
type: format
fstype: btrfs
```
just mkfs.btrfs runs on a given partition.
No possibility to run "btrfs subvolume create" before mount.

A subvolume can be mounted via mountoption "subvol=" and a typical btrfs setup mounts / to a subvolume @ rather to the fs root (subvolid=5) itself

Expected situation:
Pull btrfs from https://git.launchpad.net/curtin/tree/curtin/block/mkfs.py
and split it like zfs or lvm into two config types or create a additional type btrfs_subvolume (to avoid breaking current configurations).

btrfs_subvolume would either require a {type: format, fstype: btrfs, ...} OR {type: btrfs_subvolume, ...} as parent as subvolumes can be **nested** inside each other.

btrfs_subvolumes could be even declared "default subvolume" thous allowing mounting into @ by just issuing the mount command to easen booting from it.