Add Amazon Linux support for EPEL

Amazon Linux ships with EPEL available, but disabled. Adding support here allows for a consistent way to enable or disable the repository across RHEL-alikes. I closed the initial pull request because I didn't like relying on the osmajorversion alone, but I don't see a better way to do it at this time. We'll have to wait and see how Amazon Linux tracks RHEL7 and adjust then.