Switch CaaSP workers to a larger flavor

With the current default (m1.large, i.e. 8GB of RAM) deployments run out
of memory from time to time, with the OOM killer starting to kill
processes. Let's switch to a bigger flavor (m1.medlarge, i.e. 12GB of RAM)
to avoid that.

files/caasp-stack.yaml

@@ -81,7 +81,7 @@ parameters:
   worker_flavor:
     type: string
     description: Worker Flavor
-    default: m1.large
+    default: m1.medlarge
     constraints:
       - custom_constraint: nova.flavor
   worker_count: