[ovirt-users] Added RH EL and CentOS in engine install part of docs

Hello, I'm installing a new test oVirt environment and I'm going to use CentOS 6.5 for the engine. I just verified that the instructions at http://www.ovirt.org/Quick_Start_Guide work correctly and exactly the same for CentOS too. Previously only Fedora was referred and the first-time coming users could feel confused... so I took the time to add Red Hat Enterprise Linux and CentOS. Feel free to revise and correct the page Gianluca