Adding 'switches' to gradle

This change adds the ability to specify switches to be passed to
gradle.  This change was only two functional code lines and
several lines of documentation.  The only file that changed was
builders.py

Change-Id: Idb1199cd4a63e6fbfdabe57cf50a8b431dd9e914
Reviewed-on: https://review.openstack.org/25929
Reviewed-by: Tollef Fog Heen <tfheen@varnish-software.com>
Reviewed-by: Arnaud Fabre <fabre.arnaud@gmail.com>
Reviewed-by: Clark Boylan <clark.boylan@gmail.com>
Approved: James E. Blair <corvus@inaugust.com>
Reviewed-by: James E. Blair <corvus@inaugust.com>
Tested-by: Jenkins