Some image-properties should not be inherited from the instance when
taking a snapshot because we want that functionality to only available
for the base-image, not subsequent snapshots.
As an example, we would like 'cache_in_nova' fast-cloning to be
available just for base-images and not customer snapshots created off a
base-image.
This patch adds a new configuration which allows operators to, at
runtime, select which image-properties should not be inherited by newly
created snapshots.
Change-Id: I8d7781b05ffd71a59ba69fb7c3df4616cba94cce
dee900664b