stack/monitor: add credential check to frontend promise

This is meant to use the new HTTP basic authentication check introduced
in nexedi/slapos.toolbox!98.

stack/monitor/instance-monitor.cfg.jinja2.in

@@ -357,7 +357,9 @@ module = check_url_available
 name = monitor-http-frontend.py
 url = ${monitor-frontend:connection-secure_access}
 config-url = ${:url}
-config-check-secure = 1
+config-username = ${monitor-instance-parameter:username}
+config-password = ${monitor-instance-parameter:password}
+require-auth = 1
 
 [monitor-bootstrap-promise]
 <= monitor-promise-base