Use a local cache for JJB jobs

Use a local cache for JJB jobs

This should prevent the error:

    Traceback (most recent call last):
      File "/tmp/tmp.5aGg6za4Iw/bin/jenkins-jobs", line 8, in <module>
        sys.exit(main())
      File "/tmp/tmp.5aGg6za4Iw/lib/python3.5/site-packages/jenkins_jobs/cli/entry.py", line 165, in main
        jjb.execute()
      File "/tmp/tmp.5aGg6za4Iw/lib/python3.5/site-packages/jenkins_jobs/cli/entry.py", line 146, in execute
        ext.obj.execute(self.options, self.jjb_config)
      File "/tmp/tmp.5aGg6za4Iw/lib/python3.5/site-packages/jenkins_jobs/cli/subcommand/test.py", line 55, in execute
        options, jjb_config)
      File "/tmp/tmp.5aGg6za4Iw/lib/python3.5/site-packages/jenkins_jobs/cli/subcommand/update.py", line 97, in _generate_xmljobs
        builder = JenkinsManager(jjb_config)
      File "/tmp/tmp.5aGg6za4Iw/lib/python3.5/site-packages/jenkins_jobs/builder.py", line 60, in __init__
        flush=jjb_config.builder['flush_cache'])
      File "/tmp/tmp.5aGg6za4Iw/lib/python3.5/site-packages/jenkins_jobs/cache.py", line 54, in __init__
        "Unable to lock cache for '%s'" % jenkins_url)

Change-Id: Ia027d64aec0caf582893009e709f938531bea15d
Signed-off-by: Trevor Bramwell <tbramwell@linuxfoundation.org>