Implements bp retrieve-image-from
Added support for a new header:
x-glance-api-copy-from: URI
where URI is a HTTP, S3, or Swift location that's accessible to the
glance API service. Where present, the image content is copied from
the external store to the backend store currently configured for
the glance API service.
Impedence between the representation returned by Store.get() and
the implicit requirements of Store.add() required careful iterator/
filelike wrapping.
Also rationalized the S3 and Swift functional test setup logic and
added a new decorator-based mechanism to allow a functional testcase
require multiple backend stores to be available and configured.
Change-Id: If783a09c273832b8926aabc60b7eba8b3ab956d6
c83bce1ce6