Add possibility to enable network policy via Calico network controller

The requirements for network policy feature are described here [1]. In
order to enable it, appropriate configuration must be provided to the CNI
plug in and Calico policy controller must be set up. Beside that
corresponding extensions needed to be enabled in k8s API.

Now to turn on the feature user can define `enable_network_policy`
customization variable for Ansible.

[1] http://kubernetes.io/docs/user-guide/networkpolicies/

roles/kubernetes/node/templates/cni-calico.conf.j2

@@ -5,6 +5,11 @@
   "ipam": {
     "type": "calico-ipam"
   },
+{% if enable_network_policy is defined and enable_network_policy == True %}
+  "policy": {
+    "type": "k8s"
+  },
+{% endif %}
   "kubernetes": {
     "kubeconfig": "{{ kube_config_dir }}/node-kubeconfig.yaml"
   }