Add python unittests to JJB.

* modules/openstack_project/files/jenkins_job_builder/config/projects.yaml:
Use python-jobs group with the jenkins-job-builder project as unittests
are being added to the project.

* modules/openstack_project/files/zuul/layout.yaml: Run python unittests
against Jenkins Job Builder in the check and gate pipelines.

This depends on https://review.openstack.org/#/c/36990/

Change-Id: I945e159f1e9cbcc11a2b94e4b7ecead26ba63029
Merge "Add python unittests to JJB."