Allow to override ignore upstream changes setting

When ignore-upstream-changes is set to false, it will configure
the job to build whenever a SNAPSHOT dependency is built.

This setting is only available to Maven projects.

The default value is still 'true' for backward compatibility.

Change-Id: Ibdcdeef6264a68b9002a5b5bb35b90233db67e8a
Reviewed-on: https://review.openstack.org/32298
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