Native Zuul v3 watcher-grenade job + some cleanup

Create a native Zuul v3 grenade job. It matches the existing job,
even though it doesn't call any local hook as the current legacy
job does (because no local hook exists and it should be rewritten
as zuul configuration if it did).

The new job reuses the variable definition of the devstack watcher
job, so clean up that job as well:
- do not depend on devstack-gate, which is not needed and will be
  deprecated soon anyway;
- use the new way (tempest_plugins) to define which tempest plugin
  should be installed;
- remove the definition of USE_PYTHON3: true and simply inherit
  the value set by devstack;
- remove the definition of PYTHONUNBUFFERED, not really set
  anywhere else and only useful back in the days in Jenkins.

Change-Id: Ib0ed3c0f395e1b85b8f25f6e438c414165baab32
(cherry picked from commit ed59145354)

.zuul.yaml

@@ -160,7 +160,6 @@
     timeout: 7200
     required-projects: &base_required_projects
       - openstack/ceilometer
-      - openstack/devstack-gate
       - openstack/python-openstackclient
       - openstack/python-watcherclient
       - openstack/watcher
@@ -179,14 +178,10 @@
         s-container: false
         s-object: false
         s-proxy: false
-      devstack_localrc:
-        TEMPEST_PLUGINS: /opt/stack/watcher-tempest-plugin
-        USE_PYTHON3: true
+      tempest_plugins:
+        - watcher-tempest-plugin
       tempest_test_regex: watcher_tempest_plugin.tests.api
       tox_envlist: all
-      tox_environment:
-        # Do we really need to set this? It's cargo culted
-        PYTHONUNBUFFERED: 'true'
       zuul_copy_output:
         /etc/hosts: logs
 
@@ -200,10 +195,12 @@
 
 - job:
     name: watcher-grenade
-    parent: legacy-dsvm-base
-    timeout: 10800
-    run: playbooks/legacy/grenade-devstack-watcher/run.yaml
-    post-run: playbooks/legacy/grenade-devstack-watcher/post.yaml
+    parent: grenade
+    required-projects:
+      - openstack/watcher
+      - openstack/python-watcherclient
+      - openstack/watcher-tempest-plugin
+    vars: *base_vars
     irrelevant-files:
       - ^(test-|)requirements.txt$
       - ^.*\.rst$
@@ -215,12 +212,6 @@
       - ^setup.cfg$
       - ^tools/.*$
       - ^tox.ini$
-    required-projects:
-      - openstack/grenade
-      - openstack/devstack-gate
-      - openstack/watcher
-      - openstack/python-watcherclient
-      - openstack/watcher-tempest-plugin
 
 - job:
     # This job is used in python-watcherclient repo