Enable cross-cell resize in the nova-multi-cell job

This changes the nova-multi-cell job to essentially
force cross-cell resize and cold migration. By "force"
I mean there is only one compute in each cell and
resize to the same host is disabled, so the scheduler
has no option but to move the server to the other cell.

This adds a new role to write the nova policy.yaml file
to enable cross-cell resize and a pre-run playbook so
that the policy file setup before tempest runs.

Part of blueprint cross-cell-resize

Change-Id: Ia4f3671c40e69674afc7a96b5d9b198dabaa4224

playbooks/nova-multi-cell/pre.yaml

@@ -0,0 +1,7 @@
+- hosts: controller
+  roles:
+    # /etc/nova/policy.yaml is going to be owned by the stack user so we need
+    # to make sure that user exists on the controller host first.
+    - setup-stack-user
+    # Write rules to the /etc/nova/policy.yaml file.
+    - setup-multi-cell-policy