spec: discovery URIs are vague as to required variables

[tdavis]

The discovery spec states how URI templates must be interpolated:

MUST perform a simple template substitution to determine the URL at which the resource can be retrieved ... where name, version, os, and arch are set to their respective values for the image...

The specification is vague on whether or not those particular variables are required in the template; aside from name they are optional labels for a manifest. It also doesn't specify whether or not other arbitrary labels may be used in the template.

[jonboulle]

See #596

[tdavis]

Closing as duplicate of #596, thanks for the link!