ssh.OpenWRT: refactored manufacturer retrieval
renamed get_manufacturer_of_interfaces() in manufacturer which now returns a string instead of a list
ssh.OpenWRT: improved _filter_radio()
manage case in which there is no wireless interface available on the device
backends.dummy: deprecation notice
ssh.OpenWRT: refactoring of ubus_dict
 * renamed _ubus_dict to ubus_dict
 * cached ubus_dict