Update the RHEL installation guide
Update the RHEL installation guide

This updates the RHEL install instructions to reflect the new COPR for
installation on RHEL/CentOS 5.

Additionally, it provides better instructions on how to enable EPEL:

- We don't care about EPEL5, so this link has been removed.
- We needed a link for EPEL7, so this has been added.
- The old RPM installation example used hard-coded URLs to an arbitrary
  EPEL mirror. This is not a good idea since A) the mirror could go
  away, and B) we are unfairly sending people to a single mirror when
  the Fedora Project has a perfectly good link on their wiki to send
  people to a "random" mirror. The install example now uses these links.

saltstack/salt,saltstack/salt,saltstack/salt,saltstack/salt,saltstack/salt