972880 - The ISO importer now checks all ISO units before downloading

https://bugzilla.redhat.com/show_bug.cgi?id=972880

Before, it only checked the repository it was importing to for content that already existed on disk. This would cause it to redownload content, and if this was a sync from localhost it would overwrite the content already on disk.

One thing I seriously considered was making the importer use its working directory, but this would (I believe) require a good amount of refactoring and won't be very beneficial (other than being good practice).

This also fixes https://bugzilla.redhat.com/show_bug.cgi?id=965769
I think we should try to read this programmatically rather than duplicating the information here. If we ever change the unit key, we are likely to forget to update it here too. I don't know off the top of my head what code we have to read this, but I'd highly suggest getting it that way instead.