Change Review merge policy from two +2 to single +2 requirement

Now a days, Tempest does not have much active core reviewers due
to that merging the incoming code change is taking too much time.

As you can see we have lot of backlogs in open review ~400

If situation improve in future and we have more Core review
then we can change the policy back to two +2 requirement.

Discussion in QA office hour:
-  http://eavesdrop.openstack.org/meetings/qa/2021/qa.2021-01-26-14.00.log.html#l-147

Change-Id: Ic005de3e56cb4fde9cfce183ee598c7e9f77f464
2 files changed