build/repositories: add add_rh_cdn_repo

add_rh_cdn_repo is a new function in build/repositories used to add a
repository from CDN Red Hat channels.
This is the recommanded way to install Red Hat Enterprise Linux
OpenStack Platform 5.

Source: http://goo.gl/BCwNbU
Signed-off-by: Emilien Macchi <emilien.macchi@enovance.com>