Clean-up drive provisioning code

[Jason Gerard DeRose]

Some time ago I added the experimental `dmedia-provision-drive` script and its backing functionality in the `dmedia.drives` module. This code is for formatting a drive the "Dmedia way", plus for extracting useful metadata about a drive or partition from udev (whether or not a drive is being formatted).

Now that we're more committed to using this code, it's time to clean it up a bit and add comprehensive unit tests.

The metadata from udev has an important use when it comes to putting FileStore (drives) into a "failed" state, because we want to track this based on the drive serial number and the partition UUID, in addition to the FileStore ID.