[WFCORE-105] Enabling Kerberos authentication over the HTTP management interface.

I think 'as' is fine here - when we receive a HTTP request from a client the Host header could be any one of a number of host names - what we are doing here is selecting a keytab so that we can assume the identity of the host the client thinks they are talking to.

If no keytab is selected we can not use Kerberos as the host the client thinks we are.