fix fail with missing mtu

Change-Id: I6ccd8972ccc4cf320fb4d0abd742e96ed961478f
diff --git a/kubernetes/files/calico/calico.conf b/kubernetes/files/calico/calico.conf
index 2baa801..8b48f73 100644
--- a/kubernetes/files/calico/calico.conf
+++ b/kubernetes/files/calico/calico.conf
@@ -14,7 +14,7 @@
       "type": "k8s"
     },
 {%- endif %}
-{%- if  pool.network.mtu %}
+{%- if pool.network.get('mtu') %}
     "mtu": {{ pool.network.mtu }},
 {%- endif %}
     "log_level": "info",