Allow Registry servers to have dynamic image attributes

Image Fields

All images stored in an image registry have a number of image fields. Image attributes are typed and may have constraints put on the data they contain -- such as nullability, maximum lengths, default values, etc.

There is a set of standard image fields:

    id -- The image identifier
    name -- A string name for the image
    status -- A string representing the status of an image
    size -- An integer representing the size of the image file in bytes
    created_at -- A string when the image was registered (ISO timestamp)
    updated_at -- A string when the image metadata was modified (ISO timestamp)
    deleted_at -- A string (or null) when the image was deleted (ISO timestamp)
    disk_format -- A string representing the disk format of the image
    container_format -- A string representing the container format of the image
    owner -- A string representing the owner of the image, or null
    min_disk -- A number representing the minimum disk size needed for the image
    min_ram -- A number representing the minimum amount of memory needed to boot the image

Extended Image Fields

All image registries are required to publish the above standard image fields. However, they are free to also publish any number of additional image fields. These additional, non-standard, attributes are known as extended image fields and are discoverable in the same way as other attributes.

In Glance Registry, the implementation of extended image fields should be easy to manage and maintain. This means that extended fields should be configurable via configuration files (JSON-encoded list of extended field mappings?) and that the registry database schema should be migrated properly when these extended fields collection changes.