Move CNI config and add MTU support for calico-cni

- Move CNI configuration creation for Calico to appropriate
network_plugin role from kubernetes/node.
- Add support for MTU configuration in Calico.

roles/network_plugin/calico/defaults/main.yml

@@ -11,3 +11,8 @@ overwrite_hyperkube_cni: true
 
 calico_cert_dir: /etc/calico/certs
 etcd_cert_dir: /etc/ssl/etcd/ssl
+
+# You can set MTU value here. If left undefined or empty, it will
+# not be specified in calico CNI config, so Calico will use built-in
+# defaults. The value should be a number, not a string.
+# calico_mtu: 1500