Fix RHEL installation support

The commit 7ea6b570c3ff550f145ef98af66169bfb6500e78 broke RHEL support by introducing the parameters `package_name` and `package_source` in `package.pp` but those parameters are not supported in `package/redhat.pp`.

This pull request fixes the problem and also consolidates redhat.pp to match the style of debian.pp.  This includes replacing the `$redhat_packages` parameter (a list of packages) with simply `$package_name`.  Checking the latest nginx in EPEL for RHEL6 all related packages with the exception of libXpm are already specified as dependencies in the `nginx` RPM.  I haven't found out why `libXpm` was listed in `$redhat_packages` in the first place.