remove unnecessary dependency on argparse
remove unnecessary dependency on argparse

argparse is not required for Python 2.7 and 3.4 (which are the only
Python versions explicitely supported in setup.py)

removing the unnecessary argparse dependency to allow package to be
properly packaged on Debian/Ubuntu systems.

Change-Id: Ia268edc0c674ec3514139570d527a3f340efb1ac

openstack/stevedore