977896: Fixes for Workstation/Desktop certs

productid installation code was broken for the
case where Desktop and Workstation productid were
installed at the same time. In that case, only
the Workstation productid should be installed.

Previously, we handled the cases where those
were installed at different times.

This adds a filter step after deciding what
productis to install that removes Desktop if
both are available.
Merge pull request #699 from candlepin/alikins/977896_dt_ws_productid

977896: Fixes for Workstation/Desktop certs