SEC15 — PrincipalAuthenticator and Current

Summary: I have a problem discovering the semantics of using
Current.set_credentials(SecInvocationCredentials) as opposed to using an
InvocationCredentials Policy object on Current, or the target object
reference.

I believe we should deprecate set_credentials and get_credentials infavor
of InvocationCredentials and NonRepudiationCredentials Policy Objects.
This mechanism seems more in line with the the way we seem to be
traveling. So it will be consistent with the client invocation policy
model.

Or. we should put an explicit statement ordering the retrieval of
credentials at object reference creation.

1. InvocationCredentailsPolicy object off of Target object reference.
2. InvocationCredentialsPolicy object from the Current.
3. Current.get_credentials()

Close issue 2027: Principal Authenticator and Current