Add Swarm management backend

As described in our ROADMAP.md, introduce new Swarm management API
endpoints relying on swarmkit to deploy services. It currently vendors
docker/engine-api changes.

This PR is fully backward compatible (joining a Swarm is an optional
feature of the Engine, and existing commands are not impacted).

Signed-off-by: Tonis Tiigi <tonistiigi@gmail.com>
Signed-off-by: Victor Vieux <vieux@docker.com>
Signed-off-by: Daniel Nephin <dnephin@docker.com>
Signed-off-by: Jana Radhakrishnan <mrjana@docker.com>
Signed-off-by: Madhu Venugopal <madhu@docker.com>

api/server/httputils/errors.go

@@ -8,6 +8,7 @@ import (
 	"github.com/docker/engine-api/types"
 	"github.com/docker/engine-api/types/versions"
 	"github.com/gorilla/mux"
+	"google.golang.org/grpc"
 )
 
 // httpStatusError is an interface
@@ -58,6 +59,7 @@ func GetHTTPErrorStatusCode(err error) int {
 			"wrong login/password":  http.StatusUnauthorized,
 			"unauthorized":          http.StatusUnauthorized,
 			"hasn't been activated": http.StatusForbidden,
+			"this node":             http.StatusNotAcceptable,
 		} {
 			if strings.Contains(errStr, keyword) {
 				statusCode = status
@@ -85,7 +87,7 @@ func MakeErrorHandler(err error) http.HandlerFunc {
 			}
 			WriteJSON(w, statusCode, response)
 		} else {
-			http.Error(w, err.Error(), statusCode)
+			http.Error(w, grpc.ErrorDesc(err), statusCode)
 		}
 	}
 }