optparse has been deprecated since version 3.2 and argparse is the
blessed replacement. Take the opportunity to enhance our help output
showing defaults when called.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Robert Foley <robert.foley@linaro.org>

Message-Id: <20200701135652.1366-15-alex.bennee@linaro.org>