When swarm-mode is disabled, we need to return an error indicating this.
406 was chosen for the "Not Acceptable" verbiage, but this code has
specific semantics in relation to the `Accept` header, which aren't
applicable here.
We now use a 503 for this case. While it is not a perfect match, it does
make it clear that the particular "service" (read: API endpoint) is not
available. The body of the message provides the user with enough
information to take action on it by enabling swarm-mode and ensuring the
service is available.
Signed-off-by: Stephen J Day <stephen.day@docker.com>
1d90d76048