Make kubelet_load_modules always present but false.

Update code and docs for that assumption.

roles/kubernetes/node/templates/kubelet-container.j2

@@ -14,7 +14,7 @@
   {% for dir in ssl_ca_dirs -%}
   -v {{ dir }}:{{ dir }}:ro \
   {% endfor -%}
-  {% if kubelet_load_modules is defined and kubelet_load_modules == true -%}
+  {% if kubelet_load_modules -%}
   -v /lib/modules:/lib/modules:ro \
   {% endif -%}
   -v /sys:/sys:ro \