[openstack-ansible] avoid approving changes hitting centos-7 jobs

Hi everyone: With the release of CentOS 7.6, we are unable to merge any code which runs on CentOS 7 because of the fact that our containers are still 7.5 however the host is 7.6 There is an issue open to get CentOS 7.6 images in: https://github.com/CentOS/sig-cloud-instance-images/issues/133 We’re hoping that upstream can provide this CentOS image soon to unbreak us but until then we’ll be blocked. I’ll try to reach out to anyone on the CentOS 7 team who can help us, but for the meantime, let’s avoid approving anything with voting CentOS 7 jobs. Thanks! Mohammed