Simplify HDP repository handling for platform_family == rhel

Rather than be stringent on which versions we support, detect usage of RHEL 5 equivalents. Otherwise, use the "centos6" repository, which should support centos, redhat, and scientific versions. Amazon will be treated as RHEL 6 equivalent.
Merge pull request #102 from continuuity/feature/simplify-hdp-rhel

Simplify HDP repository handling for platform_family == rhel