Add devstack job with JSONFormatter configured

We've run into issues in the past where a service passed something
to the logger that broke JSONFormatter. To try to catch those sooner,
add a job that configures the services to use JSONFormatter. This
should provide a more realistic test of the formatter than we can
hope to accomplish in unit tests.

Change-Id: Icfb399cfe3dce89dfd5fb5079295a4947828417a

.zuul.yaml

@@ -13,12 +13,42 @@
     run: playbooks/legacy/oslo.log-src-grenade-devstack/run.yaml
     timeout: 10800
 
+- job:
+    name: oslo.log-jsonformatter
+    parent: devstack-tempest
+    timeout: 10800
+    vars:
+      devstack_local_conf:
+        post-config:
+          $NOVA_CONF:
+            DEFAULT:
+              use_json: True
+          $NEUTRON_CONF:
+            DEFAULT:
+              use_json: True
+          $GLANCE_CONF:
+            DEFAULT:
+              use_json: True
+          $CINDER_CONF:
+            DEFAULT:
+              use_json: True
+          $KEYSTONE_CONF:
+            DEFAULT:
+              use_json: True
+    irrelevant-files:
+      - ^.*\.rst$
+      - ^api-ref/.*$
+      - ^doc/.*$
+      - ^releasenotes/.*$
+
+
 - project:
     check:
       jobs:
         - oslo.log-src-grenade-devstack
-
+        - oslo.log-jsonformatter
         - openstack-tox-lower-constraints
     gate:
       jobs:
+        - oslo.log-jsonformatter
         - openstack-tox-lower-constraints