set custom ntp server for debian unattended

templates/preseed.cfg.j2

@@ -148,7 +148,11 @@ d-i time/zone string {{ debianunattended_time_zone | default('EU/Berlin') }}
 # Controls whether to use NTP to set the clock during the install
 d-i clock-setup/ntp boolean true
 # NTP server to use. The default is almost always fine here.
+{% if debianunattended_ntpserver is defined %}
+d-i clock-setup/ntp-server string {{ debianunattended_ntpserver }}
+{% else %}
 #d-i clock-setup/ntp-server string ntp.example.com
+{% endif %}
 
 ### Partitioning
 ## Partitioning example