Add a periodic fast forward upgrade job

Fast forward upgrade (FFU) is an upgrade strategy that does in-place
upgrade and skips intermediary releases (e.g. N -> N+3).

This new periodic job fast forwards a cloud from Stein to master.

Please note that ideally a distro upgrade should also be performed but
Grenade does not support that today. For example, Bionic is a tested
Linux distribution on Stein, whereas on Victoria it is Focal.

Change-Id: I792f42468c37b6849c8fc085172841434d6a1376

zuul.d/jobs.yaml

@@ -66,6 +66,12 @@
         '/var/log/octavia-tenant-traffic.log': 'logs'
         '/var/log/octavia-amphora.log': 'logs'
 
+- job:
+    name: octavia-grenade-ffu
+    parent: octavia-grenade
+    vars:
+      grenade_from_branch: stable/stein
+
 - job:
     name: octavia-v2-dsvm-scenario-amphora-v2
     parent: octavia-v2-dsvm-scenario

zuul.d/projects.yaml

@@ -80,3 +80,4 @@
         - publish-openstack-octavia-amphora-image-centos8:
             branches: ^(?!stable/.*).*$
         - octavia-amphora-image-build
+        - octavia-grenade-ffu