Improve parsers/redhat_release to detect CentOS correctly

[PaulWay]

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Complete Description of Additions/Changes:

The RedhatRelease parser already does a basic check of the operating system to set flags for whether this is a RHEL, CentOS, Fedora or unknown system. We need to test that these are handled correctly within the RedHatRelease combiner.

Part of this work simplifies the short product code detection in the RedhatRelease parser, which we now advertise as a property.

Additionally, RHEV 3.5 now correctly detects as a RHEL operating system.