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