Add an extension to show image size.

Adds image size data that is returned from Glance into the API response.
This becomes really useful for users when they take a snapshot and want
to track the storage used by it.  Since this information is already
being pulled by Nova we should display it.

DocImpact
Change-Id: I8a2d4001c21bcc54cac1a2992034dfc9fbe39d7e
Merge "Add an extension to show image size."