Only trigger JJB config diff when changed

integration-jjb-config-diff generates the Jenkins jobs twice which takes
roughly 3 minutes.  That is rather useless when there is no
modifications to any of the JJB configuration files, for example a
change to the Zuul configuration file.

Skip the job unless the change touch files under jjb/.

Change-Id: Id5a13a0bf4b7ebb9f822f6907a04a06bed146a9b