Kerberos auth support

[jflorian]

This module presently only supports x509 authentication. Koji supports both x509 and Kerberos; this module should as well.

[jflorian]

I guess the first step would be to identify what parts of Koji use Kerberos to know specifically what must be changed here. Also, my present understanding of authentication in Koji is how it uses x509. I'm assuming its use of Kerberos would be similar but awareness of any other differences would be helpful, but those can become evident as we go, if that's easier. (I intend to migrate my Koji auth in the process.)

[jflorian]

X.509 certificate usage is now optional with this Puppet module. That implementation (via the various *::x509 classes) should serve as a good model for implementing (optional) Kerberos support.