Backport the followign commit from bitcoin core:
```
commit 91bfffff5d1de034661e7fd6b319447b47ae6e33
Author: John Newbery <john@johnnewbery.com>
Date: Tue Jan 31 10:15:40 2017 -0800
Use argparse in rpc_tests.py
This commit replaces the roll-your-own argument parsing in rpc_tests.py
with Python's standard library argparse.
```