Code review comment for lp:~blake-rouse/maas/add-osystem-to-bootimage

I added test for get_usable_osystems and get_useable_releases, these methods get used in the following branches.

Andres you are correct in an issue on upgrade, this should be the only upgrade issue I can think of out of all of these branches. I don't think it would be to hard to handle, as we know the only operating system that is in boot-resources folder is Ubuntu.

So on upgrade move all folders in the boot-resources folder into a sub-folder named ubuntu, except for the grub folder it should not move. The migration already sets all BootImage items to ubuntu, so nothing more need to be done there. Only thing left to do is update the maas.tgt, with ubuntu in the path.