Fix external network name: key being ignored in get_net_args_from_networks() - #1552

…etworks()

When `external: name: custom-network` was used,
`get_net_args_from_networks()` checked `isinstance(is_ext, str)` instead
of `isinstance(is_ext, dict)`, so the dict value was discarded and
`podman-compose` looked up the auto-prefixed name instead of the
explicitly provided one.

Signed-off-by: Monika Kairaityte <monika@kibit.lt>