Implement Kerberos HTTP authentication handler

This commit implements a Kerberos HTTP authentication handler. It
uses credentials from a local cache to perform an HTTP authentication
negotiation using the GSSAPI.

The purpose of this handler is to allow the use Kerberos authentication
to access review endpoints without the need to transmit the user
password.

Change-Id: Id2c3fc91a58b15a3e83e4bd9ca87203fa3d647c8
1 file changed