Add RHEL as SL/CentOS compatible OS

Following the announcement yesterday that the LHCb environment entry point will quite soon change from LbLogin to LbEnv, I thought that I would start to test how the new system works on our local cluster in Warwick.

Our login node runs Red Hat Enterprise Linux Server 6, which the host_os script does not currently detect as being an OS compatible with SL/CentOS. I've modified the regular expression used in the script to include it.