RequestContext 'auth_tok' inconsistent with RPCContext

RequestContext holds the Auth-Token in an attributed named 'auth_tok'. When this context is serialized to amqp (pack_context), its to_dict method is called and the resulting dictionary converts the key to 'auth_token'.

After unpacking into RPCContext, the Auth-Token now is only accessible via the 'auth_token' attribute instead. So if a RPC Service intends to use it, it has to be aware of the changed attribute name.

    def to_dict(self):
        return {'user': self.user,
                'tenant': self.tenant,
                'is_admin': self.is_admin,
                'read_only': self.read_only,
                'show_deleted': self.show_deleted,
                'auth_token': self.auth_tok, <-----------------------
                'request_id': self.request_id}