maven: support for MAVEN_OPTS

The maven build phase offers us a way to pass additional parameters to
the JVM.  The option is shown as MAVEN_OPTS and accept a string which is
then expanded as mavenOpts.

Change-Id: I0a2437e8bda1e828c09e87343f7ca5025b77182c
Reviewed-on: https://review.openstack.org/32965
Reviewed-by: Arnaud Fabre <fabre.arnaud@gmail.com>
Reviewed-by: Mathieu Gagné <mgagne@iweb.com>
Reviewed-by: Khai Do <zaro0508@gmail.com>
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