sdk: Add support for Kerberos authentication

This patch adds support for Kerberos authentication. To use it the API
object has to be created using the "kerberos" parameter:

  api = ovirtsdk.api.API(
    url="https://ovirt.example.com/ovirt-engine/api",
    kerberos=True,
    ...
  )

For this to succeed the user must have acquired the Kerberos credentials
(the TGT) before calling the Python SDK.

Change-Id: Ic36e8548e5743aca71757eba46e4970976f08911
Bug-Url: https://bugzilla.redhat.com/1145234
Signed-off-by: Juan Hernandez <juan.hernandez@redhat.com>