Add argparse to requirements.txt

argparse is a requirement of os-refresh-config, so it should be listed here.
Likely this error is never encountered b/c argparse is part of python 2.7. But,
for older distros like RHEL that are still on python 2.6, it needs to know to
install it explicitly.

Change-Id: I63bd54bf42d99d2ef7f6534848afc396471cd802