Adds property to selectively enable image caching.
Adds property to selectively enable image caching.

The image-property `cache_in_nova` can be used to choose which images
should be cached in Nova. One use-case would be to cache base images but
not any customer snapshots.

Change-Id: I1b8ac914a6effd4cd4653aae7e4eac9d14d0e7bd

n0ano/gantt,n0ano/gantt