Adding root-build-script to gradle

This change adds a boolean to specify whether or not to use the
newly added root build script value.

Change-Id: Idf691c94e44e13fe5f3b03f9c2003875334b912f
Reviewed-on: https://review.openstack.org/36676
Reviewed-by: Clark Boylan <clark.boylan@gmail.com>
Reviewed-by: Arnaud Fabre <fabre.arnaud@gmail.com>
Approved: James E. Blair <corvus@inaugust.com>
Reviewed-by: James E. Blair <corvus@inaugust.com>
Tested-by: Jenkins