Switch reviewers command to use argparse
Switch reviewers command to use argparse

optparse is deprecated in python 2.7 and not available
starting in python3. This changes the reviewers command
to use argparse instead.

Change-Id: I76f270b4e998e8cf2a56c12d7bf70c0651c69d16

openstack-infra/reviewstats,openstack-infra/reviewstats