fix delete job command and add tests

change I126751e3 introduced recursive file definition feature however it broke
the ability to delete jobs.  This changes fixes it and adds a few tests for it.

This was the error when trying to delete a job:

(py27)~/jenkins-job-builder$ jenkins-jobs --conf jenkins_jobs.ini delete myjob
INFO:root:Deleting jobs in [myjob]
Traceback (most recent call last):
  File "/jenkins-job-builder/.tox/py27/bin/jenkins-jobs", line 10, in <module>
    sys.exit(main())
  File "/jenkins-job-builder/jenkins_jobs/cmd.py", line 122, in main
    execute(options, config)
  File "/jenkins-job-builder/jenkins_jobs/cmd.py", line 207, in execute
    builder.delete_job(job, options.path)
  File "/jenkins-job-builder/jenkins_jobs/builder.py", line 611, in delete_job
    self.load_files(fn)
  File "/jenkins-job-builder/jenkins_jobs/builder.py", line 576, in load_files
    if os.path.isdir(path):
  File "/jenkins-job-builder/.tox/py27/lib/python2.7/genericpath.py", line 41,
    in isdir st = os.stat(s)
TypeError: coercing to Unicode: need string or buffer, NoneType found

Closes-Bug: #1349634
Change-Id: Ib87fa497d80ba33470c049f875658a3878afb664
Merge "fix delete job command and add tests"