Create directory and file /etc/systemd/system/krb5-kdc.service.d/customdependecy.conf

[Unit]
Requires=slapd.service
After=slapd.service

systemctl daemon-reload systemctl list-dependencies krb5-kdc.service

Turns out this was already done by krb5-kdc-ldap though

root@matrix:/home/sunkan# systemctl status krb5-kdc.service 
  krb5-kdc.service - Kerberos 5 Key Distribution Center
   Loaded: loaded (/lib/systemd/system/krb5-kdc.service; enabled)
  Drop-In: /etc/systemd/system/krb5-kdc.service.d
           └─customdependecy.conf
        /lib/systemd/system/krb5-kdc.service.d
           └─slapd-before-kdc.conf
   Active: active (running) since Sun 2015-09-13 14:47:51 CEST; 43s ago
  Process: 898 ExecStart=/usr/sbin/krb5kdc -P /var/run/krb5-kdc.pid $DAEMON_ARGS (code=exited, status=0/SUCCESS)
 Main PID: 1239 (krb5kdc)
   CGroup: /system.slice/krb5-kdc.service
           └─1239 /usr/sbin/krb5kdc -P /var/run/krb5-kdc.pid

root@matrix:/home/sunkan# cat /lib/systemd/system/krb5-kdc.service.d/slapd-before-kdc.conf 
[Unit]
After=slapd.service
root@matrix:/home/sunkan#