Use native Zuul v3 tox job

Replace tox legacy job with native Zuul v3 job.

Also, remove name from project stanza, this is not needed anymore.

Change-Id: I97f852a39ad461c0ef7bef8071c18eb9eab453a1

.zuul.yaml

@@ -88,19 +88,20 @@
 # Experimental
 - job:
     name: keystone-tox-patch_cover
-    parent: legacy-base
-    run: playbooks/legacy/keystone-tox-patch_cover/run.yaml
-    post-run: playbooks/legacy/keystone-tox-patch_cover/post.yaml
-    timeout: 2400
+    parent: openstack-tox
+    description: |
+      Run test for keystone project.
+
+      Uses tox with the ``patch_cover`` environment.
     irrelevant-files:
       - ^.*\.rst$
       - ^api-ref/.*$
       - ^doc/.*$
       - ^etc/.*$
       - ^keystone/tests/unit/.*$
       - ^releasenotes/.*$
-    required-projects:
-      - openstack/requirements
+    vars:
+      tox_envlist: patch_cover
 
 # Experimental
 - job: