Make requirements check test more flexible.

* modules/openstack_project/files/jenkins_job_builder/config/requirements.yaml:
Check for existance of various requirements files and only clean them up
and attempt to pip install -r them if they exist.

Change-Id: I08201fa86f7a7cef47a1b2d34e44af1712f12687
Reviewed-on: https://review.openstack.org/28610
Approved: Jeremy Stanley <fungi@yuggoth.org>
Reviewed-by: Jeremy Stanley <fungi@yuggoth.org>
Tested-by: Jenkins