exeuntu: add devicemapper and btrfs kernel modules

[codablock]

I'm currently trying to use exe.dev to run dboxed.io boxes on your VMs. dboxed relies heavily on LVM to implement "dboxed volumes", so this is where I'm stuck at right now as it requires the devicemapper kernel modules.

Can you enable CONFIG_BLK_DEV_DM, CONFIG_DM_THIN_PROVISIONING and CONFIG_DM_SNAPSHOT in the kernel config?

I also plan to implement btrfs as an alternative backing store for my volumes, so maybe you can also enable BTRFS as well?