Gitignore: Add /etc/jenkins_jobs.ini and output/ directory.

Change-Id: I358b0a05a2a1a9936f30eef3e77b555eff1044ec
Reviewed-on: https://review.openstack.org/21908
Reviewed-by: Khai Do <zaro0508@gmail.com>
Reviewed-by: Clark Boylan <clark.boylan@gmail.com>
Approved: Monty Taylor <mordred@inaugust.com>
Reviewed-by: Monty Taylor <mordred@inaugust.com>
Tested-by: Jenkins