Add Fedora 21 support

[domcleal]

Packages are built on Koji and a run against Rackspace locally (with FOREMAN_CUSTOM_URL etc.) seems to be OK:

15:57 $ vagrant ssh f21 -c 'FOREMAN_CUSTOM_URL=http://koji.katello.org/releases/yum/foreman-nightly/Fedora/21/x86_64/ /vagrant/fb-install-foreman.bats'
 ✓ stop puppet agent (if installed)
 ✓ clean after puppet (if installed)
 ✓ make sure puppet not configured to other pm
 ✓ wait max 30 secs until network is up
 ✓ install SELinux tools
 ✓ update important system packages
 - subscribe and attach channels (skipped: Not required)
 - enable epel (skipped: Not required)
 ✓ configure repository
 ✓ install installer
 ✓ run the installer
 ✓ check for no changes when running the installer
 ✓ wait 10 seconds
 ✓ check web app is up
 ✓ check smart proxy is registered and Hammer runs
 ✓ install all compute resources
 ✓ check web app is up after CR installation
 ✓ collect important logs

18 tests, 0 failures, 2 skipped

[mmoll]

merged, thanks @domcleal!