monitor stack: do not regenerate httpd password if password file exists

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

@@ -221,7 +221,8 @@ update-command = ${:command}
 recipe = plone.recipe.command
 stop-on-error = true
 htpasswd-path = ${monitor-directory:etc}/monitor-htpasswd
-command = {{ apache_location }}/bin/htpasswd -cb ${:htpasswd-path} ${:user} ${:password}
+command = if [ ! -f "${:htpasswd-path}" ]; then {{ apache_location }}/bin/htpasswd -cb ${:htpasswd-path} ${:user} ${:password}; fi
+update-command = ${:command}
 user = ${monitor-instance-parameter:username}
 password = ${monitor-instance-parameter:password}