Fix the RHEL install script, mostly for RHEL 5.

Summary:
A bunch of various fixes for the RHEL install script.
Most of them are stylistic, one of them fixes the EPEL repo release
RPM URL for RHEL 5. (enough acronyms there?)

Test Plan:
Tried installing on Fedora (which is treated as RHEL 6 due to how we handle
being unable to find version in the script).

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D4038