Be explicit about objects in _shutdown_instance()

This adds a docstring to _shutdown_instance() making it clear that
it takes an Instance object. Also, pass an object to the method
from the test.

Related to blueprint compute-manager-objects-juno

Change-Id: Ia80c82ca0f5d8a470bb8f01f1520a82748a4f877
Merge "Be explicit about objects in _shutdown_instance()"