Use find module for checking for certificates

Also generate certs only when absent on master (rather than
when absent on target node)

roles/kubernetes/secrets/tasks/gen_certs_script.yml

@@ -40,12 +40,12 @@
   command: "{{ kube_script_dir }}/make-ssl.sh -f {{ kube_config_dir }}/openssl.conf -d {{ kube_cert_dir }}"
   environment:
     - MASTERS: "{% for m in groups['kube-master'] %}
-                  {% if hostvars[m].sync_certs|default(true) %}
+                  {% if gen_node_certs[m]|default(false) %}
                     {{ m }}
                   {% endif %}
                 {% endfor %}"
     - HOSTS: "{% for h in groups['k8s-cluster'] %}
-                {% if hostvars[h].sync_certs|default(true) %}
+                {% if gen_node_certs[h]|default(true) %}
                     {{ h }}
                 {% endif %}
               {% endfor %}"