Switch kolla_docker to rely on SHA256 for image changes

At the moment, the process to determine if an image has
changed or not relies on the Docker API which depending
on the Docker release server can return different results.

This patch addresses this issue by grabbing the SHA256
of the image before pulling (defaulting to None if it does
not exist) and then comparing it after the pull is complete
which should always be successful at determining if the
image did change or not.

The test for unknown status images is removed because this
is not a possible scenario as we do not rely on status
anymore except for failures (which are still tested).

Change-Id: Ia60a7f34420b02f50597dddb96a4c36ff3996612
Closes-Bug: #1668059