Implementation of NAPALM method open.

def open(self):
        """Implementation of NAPALM method open."""
        try:
            connection = self.transport_class(
                host=self.hostname,
                username=self.username,
                password=self.password,
                timeout=self.timeout,
                **self.eapi_kwargs
            )

            if self.device is None:
                self.device = pyeapi.client.Node(connection, enablepwd=self.enablepwd)
            # does not raise an Exception if unusable

            # let's try to run a very simple command
            self.device.run_commands(["show clock"], encoding="text")
        except ConnectionError as ce:
            # and this is raised either if device not avaiable
            # either if HTTP(S) agent is not enabled
            # show management api http-commands
            raise ConnectionException(py23_compat.text_type(ce))