Invalid node names when deploying with Heat

OpenStack Heat templates create Kubernetes nodes with invalid
hostnames. Capital letters are not allowed in the hostnames:

Unable to register node "kubernetes-node-6s8OizYe" with API server: Node "kubernetes-node-6s8OizYe" is invalid: metadata.name: Invalid value: "kubernetes-node-6s8OizYe": must match the regex [a-z0-9]([-a-z0-9]*[a-z0-9])?(\.[a-z0-9]([-a-z0-9]*[a-z0-9])?)* (e.g. 'example.com')

This patch prevents Heat generating hostnames that contain
capital letters.

cluster/openstack-heat/kubernetes-heat/kubeminion.yaml

@@ -252,6 +252,9 @@ resources:
     type: OS::Heat::RandomString
     properties:
       length: 8
+      character_classes:
+        - class: lowercase
+        - class: digits
 
   kube_minion:
     type: OS::Nova::Server