Adding Top-Level Maven Targets

Adding ability to use top-level maven targets within your build.
This is a change to the builders file and setup.py to add the new
functionality.

Change-Id: I7ae24b82d0b0ba4e81eda67045131733ced1dc46
Reviewed-on: https://review.openstack.org/25413
Reviewed-by: Khai Do <zaro0508@gmail.com>
Reviewed-by: Arnaud Fabre <fabre.arnaud@gmail.com>
Reviewed-by: James E. Blair <corvus@inaugust.com>
Approved: Jeremy Stanley <fungi@yuggoth.org>
Reviewed-by: Jeremy Stanley <fungi@yuggoth.org>
Tested-by: Jenkins