ffbs-parker-nodeconfig: Get NTP server from site

This removes a hard-coded value.
Previously, this scripts have only been used by Freifunk Braunschweig.
There hard-coding was no issue.
Now, with more communities wanting to try parker out, we have to fix
those.

The `ntp_servers` list has already been introduced by Gluon upstream.
With this change we re-use this value here.

This also means that the NTP-server for the nodes should be accessible
for the clients as well.

ffbs-parker-nodeconfig/check_site.lua

@@ -1,2 +1,3 @@
 need_string({ "parker", "config_server" })
 need_string({ "parker", "config_pubkey" })
+need_string_array({'ntp_servers'})

ffbs-parker-nodeconfig/files/lib/gluon/upgrade/996-parker-dhcp-ntp-option

@@ -1,7 +1,9 @@
 #!/usr/bin/lua
 
+local site = require 'gluon.site'
+local ntp_server = site.ntp_servers()[1]
 
 local uci = require('uci')
 local x = uci.cursor()
-x:set('dhcp', 'client' , 'dhcp_option', {'option:ntp-server,172.16.0.123'})
-x:commit('dhcp')    
+x:set('dhcp', 'client' , 'dhcp_option', {'option:ntp-server,' .. ntp_server})
+x:commit('dhcp')