Add an ironic cross-job with py38

This patch adds an ironic cross-job to help verify requirements
changes.
We need to run that on Ubuntu Focal to have Python 3.8 installed
as default interpreter.

Depends-on: https://review.opendev.org/737362

Change-Id: Ie7ba23f845bc5de2c22da21c54becf6c5a0be0a5

.zuul.d/cross-jobs.yaml

@@ -85,6 +85,15 @@
     vars:
       tox_envlist: py36
 
+- job:
+    name: cross-ironic-py38
+    parent: requirements-cross-test
+    description: Run cross-project tests on ironic with py38.
+    required-projects: openstack/ironic
+    nodeset: ubuntu-focal
+    vars:
+      tox_envlist: py38
+
 - job:
     name: cross-neutron-py36
     parent: requirements-cross-test

.zuul.d/project.yaml

@@ -33,6 +33,7 @@
         - cross-glance-py36
         - cross-horizon-py36
         - cross-horizon-npm
+        - cross-ironic-py38
         - cross-keystone-py36
         - cross-neutron-py36
         - cross-nova-py36
@@ -95,6 +96,7 @@
         - cross-glance-py36
         - cross-horizon-py36
         - cross-horizon-npm
+        - cross-ironic-py38
         - cross-keystone-py36
         - cross-neutron-py36
         - cross-nova-py36: