fix(coreos-install): Add check for BTRFS.

closes #112
I do not know of a '-q' option for mount. Also if you add -s to grep you don't need to check for the file existence. So something like this:

```sh
if [[ -n "${CLOUDINIT}" ]] && ! grep -Fqs "btrfs" /proc/filesystems; then
    if ! modprobe btrfs; then
        echo "Support for BTRFS is required to use -c" >&2
        exit 1
    fi
fi
```