Add image metadata API extension

Add image metadata API extension

This patch introduces an API extension that will expose the volume
metadata inherited from the image if the volume was created from an image.

This data will be used by Nova when booting from volume without any
image supplied.

In addition, the patch introduces a helper method on the volume.API
class for accessing the image metadata and also updates the policy.json
files to be aware of the new extension.

This patch is part of the blueprint improve-boot-from-volume

This patch also introduces DocImpact as it extends the cinder API

Change-Id: I64515cc02f8de863e65e96c66eb1fda1a1954ac7