A minor change for driver_shell test
A minor change for driver_shell test

Make mock_client.driver as a Mock of DriverManager.
This will help to avoid mistake like access attributes
not belong to the object.

Change-Id: I6ef55d308b2f15451ae83ee7c206cfafd1cab94a

NaohiroTamura/python-ironicclient,NaohiroTamura/python-ironicclient,openstack/python-ironicclient,openstack/python-ironicclient