Removed drive can't be re-added as-is

[gboudreau]

When removing a drive from the pool, using --going, the .greyhole_uses_this file is renamed .greyhole_used_this.
When re-adding that drive to the pool later on (after a fsck.ext for example), the drive will simply be ignored, without any error.

Issue #25 should fix this, since it will deprecate .greyhole_uses_this files.
But we still need to make sure removed drives can be re-added as-is later.

[gboudreau]

Per GH-25 implementation, drives that are removed can be re-added later without problems, and don't require any action other than re-mounting the partition where is was previously mounted.

If the drive was changed, you'll need to execute:
greyhole --replace
to tell Greyhole that the drive was replaced.