The `file` command used here to check whether a device is formatted is not
available for CoreOS. The effect is that the mounter tries to mount an
unformatted volume which fails. This makes it quite tedious to use persistent
volumes in CoreOS.

This patch replaces the `file` command with `lsblk` which is available in
CoreOS. I checked that it's also available on RHEL, Debian, Ubuntu and SLES.