[nova][placement] introducing review priority label in gerrit

Hi, As we discussed yesterday in the retro we would like to start using the review priority label in gerrit as a replacement of the runway process. Sean proposed the project config changes for this[1]. I added couple of questions / suggestions to that patch to start a discussion about the process. Let's try to agree in the review. Then I will document the process in tree. Cheers, gibi [1] https://review.opendev.org/c/openstack/project-config/+/787523