Write KRB5REALM to /etc/sysconfig/krb5kdc and make use of common backup_config_and_replace_variables() tool
systemd service unit for krb5kdc in Fedora 16 uses KRB5REALM variable of
/etc/sysconfig/krb5kdc to start krb5kdc for the default realm. Thus, we
need to make sure it is always existing and pointing to our realm.
Partial fix for:
https://fedorahosted.org/freeipa/ticket/1192