Allow txt update for letsencrypt dns verification

[simonbuehler]

may be a good idea to add a update-policy so certbot's dns plugin can do its magic
bind/include/application/inc_bind.php

//fwrite($fh, "\tallow-update { none; };\n");
fwrite($fh, "\t update-policy { grant certbot.key name _acme-challenge." . $domain["domain_name"] . ". txt;   };\n");