Allow Dogtag RestClient to perform requests without logging in
    
    Currently the Dogtag RestClient '_ssldo' method requires a session
    cookie unconditionally, however, not all REST methods require a
    session: some do not require authentication at all, and some will
    authenticate the agent on the fly.
    
    To avoid unnecessary login/logout requests via the context manager,
    add the 'use_session' keyword argument to '_ssldo'.  It defaults to
    'True' to preserve existing behaviour (session required) but a
    caller can set to 'False' to avoid the requirement.
    
    Part of: https://fedorahosted.org/freeipa/ticket/6260
    Part of: https://fedorahosted.org/freeipa/ticket/3473
    
    Reviewed-By: Martin Babinsky <mbabinsk@redhat.com>