Align internal image API with volume and network

First part of standardizing the nova internal image API with the API
objects for volume and network. This patch creates a nova.image.api.API
object that matches the signature of nova.network.api.API and
nova.volume.cinder.API for CRUD operations on image records. The
existing unit tests were enough to fully cover the new code paths, since
much of this patch simply aligned the internal image API calls and API
object with that of the other submodules.

The next in this patch series will work on cleaning up the remainder of
places in nova that directly call on nova.image.glance methods, as well
as the untouched download/upload handling.

Change-Id: I67e7c9d841e5bad82cdca8f7048eaa57f0cfbb2f
Partially-implements: blueprint standardize-nova-image
Merge "Align internal image API with volume and network"