Split environ variables for OS name and version.

Only the ``subscribe`` method really requires the environment variable
that contains the name for the distribution where Satellite 6 will be
installed. Since only **RHEL** is supported for Satellite 6 installs, I
think it is safe to change the expected environment variables to be the
following:

* ``OS_NAME``: most of the time this should be ``rhel``
* ``OS_VERSION``: Major version for one of the supported versions
  of **RHEL**; in this case 6 or 7.
The default version should be documented...