nd_service_registry doesn't work on python3

[bemsha]

Deployment targets: Python 2.7, Python >= 3.3
gflags needs to be replaced because there's no plan to port it to Python 3. Replace it with argparse.
Python 3.1 and 3.2 don't have the "u" operator for strings, so we'll skip them.

Caveat: With argparse, unlike gflags, a Boolean flag always needs an argument.