maven-target: Expose maven version and pom location.

These parameters are configurable in Jenkins, and should
be in Jenkins Job Builder too.

Change-Id: Ia6ac92cf687ef36a99140c65dfd4bc7f7162ecf8
Reviewed-on: https://review.openstack.org/27964
Reviewed-by: James E. Blair <corvus@inaugust.com>
Approved: Clark Boylan <clark.boylan@gmail.com>
Reviewed-by: Clark Boylan <clark.boylan@gmail.com>
Tested-by: Jenkins