Define our own Centos-Base.repo during pre_suite setup

This patch provides a known Centos-Base.repo that will override the system
one to ensure Centos-Base.repo is using our preferred settings.

In particular this provides a fix for our current Centos 5 image
(ami-4433bf74) which only had 1 mirror defined. By providing the endorsed
Centos mirrorlist and baseurl we should avoid transient errors due to mirror
failures.

The configuration for these repos (and epel) have been moved into their own
setup step, to seperate them from the setup of Puppet.

Signed-off-by: Ken Barber <ken@bob.sh>