Split repository from packages

Moved add_REPO kind of function to its own file that is sourced by
packages, this way we can provide meaningful add_REPO functions.
Rename get_mirror and get_openstack_mirror to standardize functions name

Mirror meant repository so this commit standardize functions name
get_mirror() has been renamed add_main_repository()
get_openstack_mirror() has been renamed get_openstack_repository()