Prior this commit we only had support for username/password authentication
in the koji integration. This wasn't particularly useful because this
auth type isn't used in any production instance.
This commit adds the support for GSSAPI/Kerberos authentication.
The implementation uses kerby library which is very lightweight wrapper
around C gssapi library.
Also, the koji unit test and the run-koji-container script were modified
so the GSSAPI auth is fully tested.
