Assumption about service CIDR being /16 is wrong

[itamarst]

E.g. @farcaller had /13 on this custom Kubernetes setup.

Some options:

1. @farcaller proposed allowing user to customize this with env variable (Allow to override service net when heuristics fail. #238).
2. Could add command-line option.
3. Improve detection by making it more conservative. This could be done by being much more conservative, and assuming services don't get added much over time so doing minimal range that would cover existing services.
4. Improve detection by making it more accurate. Unfortunately at the moment getting this information out of Kubernetes is hard, but that might change in the future. (Expose the service-cluster-ip-range CIDR through the API Server kubernetes/kubernetes#25533).

Next steps: investigate 4. If that doesn't work, combination of 2 and 3 should do the trick. I prefer not using env variables just for consistency's sake, and would prefer DTRT, or at least mostly DTRT, over requiring user to figure out obscure information.