Updates install_jenkins_slave.sh to support RHEL.

Add support configuring Red Hat 6 jenkins slaves:

-installs EPEL
-installs puppet 2.7 (from puppetlabs repo)
-installs git

Tested on Red Hat 6.1 and 6.4.

Change-Id: Ib5808ea6356ebaa271170f68b78a431f2d68d995
Reviewed-on: https://review.openstack.org/18907
Reviewed-by: Clark Boylan <clark.boylan@gmail.com>
Reviewed-by: Jeremy Stanley <fungi@yuggoth.org>
Reviewed-by: James E. Blair <corvus@inaugust.com>
Approved: James E. Blair <corvus@inaugust.com>
Tested-by: Jenkins