Add zuulv3 main.yaml

This adds a simple starter zuulv3 main.yaml file for testing the POC
zuul deployment. Note that puppet-zuul copies the entirety of
project-config/zuul into /etc/zuul/layout on the hosts it deploys
zuul to. So this will give us /etc/zuul/layout/main.yaml which we
probably want to change to /etc/zuul/main.yaml in the future.

Change-Id: I028d3da2248ea6a28201d9afe86240b61d6cc772

zuul/main.yaml

@@ -0,0 +1,8 @@
+- tenant:
+    name: openstack
+    source:
+      gerrit:
+        config-repos:
+          - openstack-infra/project-config
+        project-repos:
+          - openstack-infra/zuul