Add F21 release to yumrepoinfo.conf. Please note that supported = yes is incorrect according to the documentation. But upgradepath uses this key to filter for 'everything but EOL and Rawhide'. We will need to overhaul this somehow, but it will be a more complex change and I didn't want to include it here. So I used supported = yes temporarily (no other check uses it now) and will file a ticket about this.

Tested with the test suite and manual execution of upgradepath on f21-updates.