Add multinode base job

Right now just does the known hosts thing

Change-Id: I3435eee42c9fc77fd3e9926477a3a261aa387eb4

playbooks/multinode/pre.yaml

@@ -0,0 +1,3 @@
+- hosts: all
+  roles:
+    - multi-node-known-hosts

zuul.yaml

@@ -134,6 +134,13 @@
       at least a ``username`` and ``password`` attribute.
     post-run: playbooks/python/upload-pypi
 
+- job:
+    name: multinode
+    description: |
+      Do additional setup needed for multi-node jobs such as setting up
+      overlay networks and setting up known-hosts and ssh keys
+    pre-run: playbooks/multinode/pre
+
 - project:
     name: openstack-infra/zuul-jobs
     check: