Before, `jenkins-job-builder test -o outdir` would write
outdir/job_name
With the new option `--config-xml`, it writes
outdir/job_name/config.xml
to match the layout that Jenkins uses internally. This is more useful
for diffing against existing Jenkins installations or uploading job
definitions by other means.
Also, it seems better to give the output files an .xml extension.
Change-Id: I78fc7aff531925ee071b8e19f40f766b4af0b5ad