Allow $RHOS to be overridden
Allow $RHOS to be overridden

This is needed to enable building images without RDO and epel.

Change-Id: I73d304d64b78376409756cef22dcc12c6d6042fb

eprasad/instack-undercloud,dmsimard/instack-undercloud,dpkshetty/instack-undercloud,rdo-management/instack-undercloud,eprasad/instack-undercloud,bcrochet/instack-undercloud,dmsimard/instack-undercloud,rdo-management/instack-undercloud,dpkshetty/instack-undercloud,bcrochet/instack-undercloud