Improved Centos 5 x86_64 image

I've taken the old ami (ami-4433bf74) and made 2 improvements:

* /etc/yum.repos.d/Centos-Base.repo now uses a mirror list
* I've installed the yum-fastestmirror package

This is primarily so we aren't just relying on the rightscale mirror. With
this setup we should reduce the transient failures due to that mirror being
unavailable for whatever reason.

Signed-off-by: Ken Barber <ken@bob.sh>