allow to suppress epel on red hat distros

The `use_upstream_package_source` parameter already allows
apt users to suppress upstream repos. After this commit,
the param also applies to red hat and centos users.

New unit tests:

    docker
      if running on a RedHat based distro
        by default
          should contain Class[epel]
        with no upstream package source
          should not contain Class[epel]
      with no upstream package source
        should not contain Class[epel]
Merge pull request #30 from jumanjiman/epel

allow to suppress epel on red hat distros