repositories: Adding add_satellite_channel

This patch add a add_satellite_channel() function to add a single
channel. This way it's much more easier to add new channel without
hacking add_satellite_repository().
openstack-common: Split Redhat & Centos configuration

When building Redhat OS we need to use the specific repostories to gain
Openstack while we shall use RDO for Centos.

This commit is making the split.
openstack-common: Adding puppet on Redhat images

Since Redhat & Centos are now split, let's use the same function to get
puppet.
openstack-full: Adding RHN Openstack repository

As we unregister from the RHN between roles, we need to reenable it
before installing openstack packages.