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

REVIEWING.rst

@@ -160,13 +160,11 @@ fix it. When it will happen, we will deal with it on a case-by-case basis.
 When to approve
 ---------------
 * It's OK to hold off on an approval until a subject matter expert reviews it.
-* Every patch needs two +2's before being approved.
-* However, a single Tempest core reviewer can approve patches without waiting
-  for another +2 in the following cases:
+* Every patch needs at least single +2's before being approved. A single
+  Tempest core reviewer can approve patches but can always wait for another
+  +2 in any case. Following cases where single +2 can be used without any
+  issue:
 
-  * If a patch has already been approved but requires a trivial rebase to
-    merge, then there is no need to wait for a second +2, since the patch has
-    already had two +2's.
   * If any trivial patch set fixes one of the items below:
 
     * Documentation or code comment typo
@@ -187,7 +185,4 @@ When to approve
     voting ``tempest-tox-plugin-sanity-check`` job) and unblock the
     tempest gate
 
-  Note that such a policy should be used judiciously, as we should strive to
-  have two +2's on each patch set, prior to approval.
-
 .. _example: https://review.opendev.org/#/c/611032/

doc/source/contributor/contributing.rst

@@ -43,10 +43,9 @@ More info about Launchpad usage can be found on `OpenStack docs page
 
 Getting Your Patch Merged
 ~~~~~~~~~~~~~~~~~~~~~~~~~
-All changes proposed to the Tempest require two ``Code-Review +2`` votes from
-Tempest core reviewers before one of the core reviewers can approve the patch by
-giving ``Workflow +1`` vote. More detailed guidelines for reviewers are available
-at :doc:`../REVIEWING`.
+All changes proposed to the Tempest require single ``Code-Review +2`` votes from
+Tempest core reviewers by giving ``Workflow +1`` vote. More detailed guidelines
+for reviewers are available at :doc:`../REVIEWING`.
 
 Project Team Lead Duties
 ~~~~~~~~~~~~~~~~~~~~~~~~