REVIEW ONLY: fixes #1685 - GSS-TSIG support for DNS updates

Based on patch by theforemanuser123 <oliver_weinmann@gmx.de>.

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Posting for review as an alternative to GH-28.  I found the kinit/klist method rather fragile when testing against FreeIPA (which also uses GSS-TSIG), so this is an alternative using the krb5_auth library.  It uses the djberg96-krb5-auth fork (the more actively maintained one) to request Kerberos credentials using the keytab and load them into the credential cache.

I've got a spec for EL6 for the rubygem here: https://github.com/domcleal/foreman-rpms/commit/76f424a8 (@skottler), though we'll use a proper release instead of patching 0.9.0 or using :git in the Gemfile.