Centos 7 Scripts

I've added in the things necessary for Centos 7 to be built using packer.
In adding this, there was some further cleanup in the openstack-yum script for the centos 6 images.

The only outstanding issue is that epel-7 currently resides in ```/7/beta/$basearch``` and so an rpm must be installed in order to not mess up the epel script for centos 6. Once epel7 moves out of the beta folder, the rpm installation step will be removed.