Add 'single core approve' policy for plugins sanity jobs failure

We discussed in Denver PTG[1] to make plugins
sanity jobs as voting which will help to mantain
the active plugins and blacklist the faulty plugins.

But when we get faulty plugins this job which will
be voting after https://review.opendev.org/#/c/641188/
can block the Tempest gate. So adding the process of
single core approve policy for mergin the patches which
will fix or remove the faulty plugins to unblock the gate.

[1] L196 https://etherpad.openstack.org/p/qa-train-ptg

Change-Id: Id38e6270ac1bae8ba63761f9bab6f1e5d0a2252b

REVIEWING.rst

@@ -183,6 +183,9 @@ When to approve
     * the project's PTL +1's the change
     * the patch does not affect any other project's testing gates
     * the patch does not cause any negative side effects
+  * If fixing and removing the faulty plugin (which leads to fail
+    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.