V4Config behavior for V4Config('10.0.0.0/24', gateway='10.0.0.1') not optimally debuggable

This leads to:

File "/usr/lib/python3/dist-packages/mako/runtime.py", line 947, in _exec_template
callable_(context, *args, **kwargs)
File "network_base_mako", line 63, in render_body
File "default_network_mako", line 74, in render_network
AttributeError: 'NoneType' object has no attribute 'prefixlen'

which is correct but a little surprising since it looks like the first arg to V4Config is going to go into 'network' since that's the first field in V4Config.

But actually it goes into dns_servers because of L3ConfigMixin

I'd love to see it reject the bare arg and/or give a better error check from or before the template