Adds help for installing prerequisites on RHEL
Adds help for installing prerequisites on RHEL

Certain prerequisites are not found by yum by default on RHEL. The yum
command specified in the RHEL instructions for installing prerequisites
fails to install python-pip and git-review. Adding a sentence directing
readers to use the EPEL software repository which has the missing
packages.

Closes-Bug: 1392355
Change-Id: Ibcbe9ff9e5d8eeb9f2511915d67dc53ac1dc1b63

bacaldwell/ironic,dims/ironic,pshchelo/ironic,rackerlabs/ironic,ionutbalutoiu/ironic,naterh/ironic,rdo-management/ironic,supermari0/ironic,redhat-openstack/ironic,pshchelo/ironic,NaohiroTamura/ironic,Tehsmash/ironic,ramineni/myironic,NaohiroTamura/ironic,dims/ironic,hpproliant/ironic,bacaldwell/ironic,naototty/vagrant-lxc-ironic,SauloAislan/ironic,openstack/ironic,naototty/vagrant-lxc-ironic,ionutbalutoiu/ironic,SauloAislan/ironic,devananda/ironic,openstack/ironic,Tan0/ironic