domain-level agnostic keytab retrieval in httpinstance
apache keytab is now retrieved using the same method in both domain levels.
The difference lies in the authentication scheme used to retrieve service
keytab:
* in DL0 passed in DM credentials are used
* in DL1 GSSAPI is used
https://fedorahosted.org/freeipa/ticket/6405
Reviewed-By: Stanislav Laznicka <slaznick@redhat.com>