Enable horizon on Fedoras

The previous apache/horizon a688bc65104e7d8998ffdb321d67a9f33df6997e
change left horizon disabled on Fedora.

Closes-Bug: #1326126

Change-Id: I79f84bb63471f84c6b37538e8dfe2bcca3ce3eac
Merge branch 'review/afazekas/f20-enable-horizon' into dt-test
Merge branch 'cinder-backends' into dt-test
Merge branch 'usb-prereq' into dt-test
Merge branch 'conf-tempest' into dt-test