Refine the image API endpoints

The image APIs was designed to pull/delete image at individual compute node but these APIs caused a lot of confusions to our users, for example https://bugs.launchpad.net/zun/+bug/1769021 .

This BP proposes to redesign the image API as below.

* List images: GET /images?host=<host>
* Get an image: GET /images/<id>?host=<host>
* Pull image: POST /images -d '{"host": ...}'
* Delete image: DELETE /images/<id>?host=<host>

NOTE: 'host' is a mandatory parameter for above endpoints.

The default policy should disallow normal users to access those APIs since only admin can manage individual host.