Add grenade-skip-level job

This adds a grenade job that specifically skips the last release,
to prove that we can do it. Such a job would facilitate a new
goal of a tick/tock release arrangement where slower-moving operators
could stick on the "tick" release and know that they can upgrade to
the next "tick" release directly.

Currently this tests from wallaby to yoga (master) and is added as
non-voting until the TC resolution passes (if it does).

Change-Id: I5a8fcc1097956ebbbe7b5ea0bed0889eeeca1d80

.zuul.yaml

@@ -9,6 +9,8 @@
         - octavia-grenade
         - ironic-grenade:
             voting: false
+        - grenade-skip-level:
+            voting: false
     gate:
       jobs:
         - grenade
@@ -366,3 +368,17 @@
       Basic multinode grenade job
     parent: grenade
     nodeset: openstack-two-node-focal
+
+- job:
+    name: grenade-skip-level
+    description: |
+      Grenade job that skips a release, validating that deployers can skip
+      specific releases as prescribed by our process.
+    parent: grenade
+    vars:
+      # Move this forward when master changes to a new skip-level-allowed
+      # target release. Right now, this is wallaby because master is yoga.
+      # When master is A, this should become yoga, and so forth.
+      grenade_from_branch: stable/wallaby
+      grenade_localrc:
+        NOVA_ENABLE_UPGRADE_WORKAROUND: True