Ensure passwordless ssh with stack between nodes

For compute migration to work, the stack user needs to be configured
with passwordless ssh between all hosts involved in the migration.

Reuse the build ssh-key for this, which is already distributed for
user root.

Depends-on: https://review.openstack.org/563584

Change-Id: Id07f55fea06509466add35315c135dbfba6aa714

roles/orchestrate-devstack/tasks/main.yaml

@@ -6,6 +6,12 @@
 - name: Setup devstack on sub-nodes
   block:
 
+  - name: Distribute the build sshkey for the user "stack"
+    include_role:
+      name: copy-build-sshkey
+    vars:
+      copy_sshkey_target_user: 'stack'
+
   - name: Sync CA data to subnodes (when any)
     # Only do this if the tls-proxy service is defined and enabled
     include_role: