stack/caucase: Allow empty on-renew

For some cases on-renew is not needed, so allow empty one in case if it's
not provided.

stack/caucase/buildout.hash.cfg

@@ -15,4 +15,4 @@
 
 [caucase-jinja2-library]
 filename = caucase.jinja2.library
-md5sum = 6394dadda5726571149ec1b21207311b
+md5sum = 93dae9ec9df9c095f69b175e5772677e

stack/caucase/caucase.jinja2.library

@@ -102,7 +102,7 @@ command-line = '{{ buildout_bin_directory }}/caucase-updater'
   --ca '{{ ca_path }}'
   --crl '{{ crl_path }}'
   {% if key_path %}--key '{{ key_path }}' {%- endif %}
-  --on-renew '{{ on_renew }}'
+  {% if on_renew %}--on-renew '{{ on_renew }}' {%- endif %}
   {% if max_sleep %}--max-sleep '{{ max_sleep }}' {%- endif %}
 {%- endmacro %}