slim down CentOS/RHEL box builds with updated ks.cfg

[juliandunn]

A while ago @dysinger showed me his ks.cfg for CentOS box builds which he said reduces the size of these boxes down to ~150 MB:

https://github.com/dysinger/basebox/blob/master/http/centos.cfg

We should adopt some of these tips about not installing all of Base, trashing the firmware bits, and other things like that.

[juliandunn]

Fixed by PR#183.