This adds an additional "Swarm" header to the _ping endpoint response,
which allows a client to detect if Swarm is enabled on the daemon, without
having to call additional endpoints.
This change is not versioned in the API, and will be returned irregardless
of the API version that is used. Clients should fall back to using other
endpoints to get this information if the header is not present.
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>
adf4bf772d