jenkins: actually enable verbose mode for git plugin

The Jenkins git plugin has a hidden verbose mode which can be enabled
via a java system property: hudson.plugins.git.GitSCM.verbose=true

In 7d3f489b, I have added a switch to JENKINS_ARGS but that made it
passed to jenkins and not to java.  This patch moves the -D definition
to JAVA_ARGS to make it an actual system property.

Change-Id: I3a7465ab3b3dec75cda10f154d9d18b13161ba3f