API comments mismatch for get_ports_from_devices

[yalei wang]

get_ports_from_devices in ml2/plugin.py will return a emtry dict when the secgroup associated is empty, due to this line
        ports = db.get_ports_and_sgs(port_ids)

but refer to the comment in neutron/db/securitygroups_rpc_base.py, seems it should at least return a dict whose secgroup's value is emtry

    def get_port_from_device(self, device):
        """Get port dict from device name on an agent.

        Subclass must provide this method or get_ports_from_devices.

        :param device: device name which identifies a port on the agent side.
        What is specified in "device" depends on a plugin agent implementation.
        For example, it is a port ID in OVS agent and netdev name in Linux
        Bridge agent.
        :return: port dict returned by DB plugin get_port(). In addition,
        it must contain the following fields in the port dict returned.
        - device
        - security_groups
        - security_group_rules,
        - security_group_source_groups
        - fixed_ips

[yalei wang (yalei-wang)]

this is a bug in get_ports_and_sgs, has been fixed in https://review.openstack.org/138633