(FACT-644) Cherry-pick lost XCP operatingsystem fact fix back into master

This commit cherry-picks work done on the operatingsystem fact that
was lost during the facter-2 / master merge.

Original commit (ed28b9382):
Ticket #18175 Wrong operatingsystem reported for Xen Cloud Platform

Facter will now return XCP for the fact operatingsystem when
/etc/redhat-release contains the XCP version string.