Add cross-nova-pep8 job

Both cinder and nova CI got broken by a recent mypy bump [0],[1].
We can try to catch these issues before the requirements update gets
merged. See also [2] for the corresponding cinder test job.

[0] https://review.opendev.org/c/openstack/cinder/+/875534
[1] https://review.opendev.org/c/openstack/nova/+/878693
[2] https://review.opendev.org/c/openstack/requirements/+/878690

Depends-On: https://review.opendev.org/c/openstack/nova/+/878693
Change-Id: I737a50e7efa2c67987f08aebc3ec0b0448ad73e5

.zuul.d/cross-jobs.yaml

@@ -191,6 +191,14 @@
     vars:
       tox_envlist: functional-py310
 
+- job:
+    name: cross-nova-pep8
+    parent: requirements-cross-test
+    description: Run cross-project tests on nova with pep8.
+    required-projects: openstack/nova
+    vars:
+      tox_envlist: pep8
+
 - job:
     name: cross-nova-py310
     parent: requirements-cross-test

.zuul.d/project.yaml

@@ -49,6 +49,7 @@
         - cross-masakari-py310
         - cross-mistral-py310
         - cross-neutron-py310
+        - cross-nova-pep8
         - cross-nova-py310
         - cross-placement-py310
         - cross-osvif-py3
@@ -123,6 +124,7 @@
         - cross-manila-py310
         - cross-masakari-py310
         - cross-neutron-py310
+        - cross-nova-pep8
         - cross-nova-py310:
             files: ^upper-constraints.txt$
         - cross-placement-py310: