allow non existing etcd group (#6797)

When using kubeadm managed etcd, configuring an etcd group can now
be skipped.

roles/network_plugin/calico/templates/calico-node.yml.j2

@@ -18,8 +18,9 @@ spec:
       labels:
         k8s-app: calico-node
       annotations:
-        # Mark pod as critical for rescheduling (Will have no effect starting with kubernetes 1.12)
+{% if calico_datastore == "etcd" %}
         kubespray.etcd-cert/serial: "{{ etcd_client_cert_serial }}"
+{% endif %}
 {% if calico_felix_prometheusmetricsenabled %}
         prometheus.io/scrape: 'true'
         prometheus.io/port: "{{ calico_felix_prometheusmetricsport }}"