Enforce linear strategy for orchestrate-devstack

Document that orchestrate-devstack requires a linear strategy in the
invoking play. Also enforce the strategy in devstack.yaml.

Change-Id: Ia081225ec2be959fc5a4ddfd491f526296a8ca10

playbooks/devstack.yaml

@@ -1,3 +1,7 @@
 - hosts: all
+  # This is the default strategy, however since orchestrate-devstack requires
+  # "linear", it is safer to enforce it in case this is running in an
+  # environment configured with a different default strategy.
+  strategy: linear
   roles:
     - orchestrate-devstack

roles/orchestrate-devstack/README.rst

@@ -15,6 +15,7 @@ data to sub-nodes and run devstack there. The only requirement for
 this role is for the controller inventory_hostname to be "controller"
 and for all sub-nodes to be defined in a group called "subnode".
 
+This role needs to be invoked from a playbook that uses a "linear" strategy.
 
 **Role Variables**