Docker

This pull request attempts to round out support for generating centos 6.5 and 7.0 boxes with docker support.

I noticed that there was a Makefile stub for creating centos65-docker boxes, but no corresponding centos65-docker.json configuration file, so I've created JSON config files for 6.5 and 7.0

I've also created a new script/epel.sh script to bootstrap the EPEL repository programmatically (this allows for installing epel-release on 6.x or 7.x without needing to download a hardcoded epel-release RPM version).  As script/docker.sh appears to be the only post-install script that relies on EPEL, I figured it was appropriate to include epel.sh in this pull request.

I've done test builds of 6.5 and 7.0 locally and everything appears to be working.