Add plugin cross jobs

This commit adds two types of cross repository jobs: python unit test and
npm test. The initial commit covers the following plugins as non-voting jobs.
They can be used as templates if more projects are tested as cross jobs.

* manila-ui for python unit test
* ironic-ui for npm test

Note that ironic-ui python unit test has only one test which checks
the panel is registered expectedly, so I don't think we need to cover
ironic-ui as a cross python unit test job.

Change-Id: I569097d6088a99542f3694e18c8d00a2b0dcf9a1

.zuul.d/cross-jobs.yaml

@@ -0,0 +1,25 @@
+- project-template:
+    name: horizon-cross-jobs
+    check:
+      jobs:
+        - horizon-cross-manila-ui-python:
+            voting: false
+        - horizon-cross-ironic-ui-npm:
+            voting: false
+
+- job:
+    name: horizon-cross-manila-ui-python
+    parent: openstack-tox
+    vars:
+      tox_envlist: py38
+      zuul_work_dir: "{{ zuul.projects['opendev.org/openstack/manila-ui'].src_dir }}"
+    required-projects:
+      - openstack/manila-ui
+
+- job:
+    name: horizon-cross-ironic-ui-npm
+    parent: horizon-nodejs10-run-test
+    vars:
+      zuul_work_dir: "{{ zuul.projects['opendev.org/openstack/ironic-ui'].src_dir }}"
+    required-projects:
+      - openstack/ironic-ui

.zuul.d/project.yaml

@@ -1,6 +1,7 @@
 - project:
     templates:
       - check-requirements
+      - horizon-cross-jobs
       - horizon-nodejs10-jobs
       - horizon-non-primary-django-jobs
       - openstack-lower-constraints-jobs