Maven: Fix XML markup.

The generated markup contains a node "root_module", which is not
recognized by Jenkins:

  WARNING: Skipping a non-existent field root_module
  com.thoughtworks.xstream.converters.reflection.MissingFieldException: No
  field 'root_module' found in class 'hudson.maven.MavenModuleSet'

The correct spelling is rootModule.

Change-Id: I0cae2c3358f2664783c97e245b064453a05bc988
Reviewed-on: https://review.openstack.org/26052
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