Fix bug in create_if_needed

Closes #12.

The logic is as follows:

* If the FS is "none", there is NOTHING to format. Period.
* If the FS is something else, we format it IFF it the block device doesn't have it yet.

As discussed on Friday, the new feature to enforce the filesystem (e.g, to (optionally) re-format an ext4 device into XFS if the profile states so) is a different improvement, that will be welcome when it gets tests.