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
diff --git a/REVIEWING.rst b/REVIEWING.rst
index e07e358..4c63aa0 100644
--- a/REVIEWING.rst
+++ b/REVIEWING.rst
@@ -160,13 +160,11 @@
 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 @@
     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/