lxc-ubuntu: use btrfs subvolumes and snapshots

Try to create the cache rootfs as a btrfs subvolume, and use btrfs
snapshots to copy the rootfs if btrfs is selected as backing store.

Signed-off-by: José Martínez <xosemp@gmail.com>
Merge pull request #244 from xose/btrfs

lxc-ubuntu: use btrfs subvolumes and snapshots