Previously an endpoint registered using `RegisterDefaultStoreEndpoints` would
not be taken into consideration by `resolveDefaultContext` and so could not
provide any details.
Resolve this by passing a `store.Config` to `resolveDefaultContext` and using
it to iterate over all registered endpoints. Any endpoint can ensure that their
type implements the new `EndpointDefaultResolver` in order to provide a default.
The Docker and Kubernetes endpoints are special cased, shortly the Kubernetes
one will be refactored to be dynamically registered.
Signed-off-by: Ian Campbell <ijc@docker.com>
1433e27420