euci: add support for ip address parsing
Note that user can use dtype=IPv6Address or dtype=IPv4Address
the result will be IPv4Address or IPv6Address regardless of dtype
settings.
Perhaps it would be better to change the api of the dtype to match standard python's argparse.
parser.add_argument(..., type=int) # type is typing.Callable[[typing.Any],typing.Any]So user could write his own convertor functions.