make-release: use argparse, snapshot by default

argparse is way funnier than getopt and gives you a nice usage message
whenever needed. Since the positional arguments are now named
(newversion, previousversion) we no more need to use args[x]:

 args[0]  => options.newversion
 args[1] => options.previousversion

The usage is now in the docstring which is passed to argparse
description. That will show up whenever one use --help

--snapshot has been removed, invoking the script with no newversion will
simply generate a snapshot.

Change-Id: I31bb325027afef2e1faf8a253ef57fbb8aecd553