dsinstance: use keytab retrieval method from parent class
DS replica can now use remote API and ipa-getkeytab to create service
principal and fetch the keytab in both domain levels. There is no need to use
KDC installer to do it.
https://fedorahosted.org/freeipa/ticket/6405
Reviewed-By: Stanislav Laznicka <slaznick@redhat.com>