Revert "use builtin package context rather than vendored one"

Signed-off-by: Flavio Crisciani <flavio.crisciani@docker.com>

cluster/provider.go

@@ -1,9 +1,8 @@
 package cluster
 
 import (
-	"context"
-
 	"github.com/docker/docker/api/types/network"
+	"golang.org/x/net/context"
 )
 
 const (

cmd/dnet/dnet.go

@@ -2,7 +2,6 @@ package main
 
 import (
 	"bytes"
-	"context"
 	"encoding/json"
 	"errors"
 	"fmt"
@@ -37,6 +36,7 @@ import (
 	"github.com/docker/libnetwork/options"
 	"github.com/docker/libnetwork/types"
 	"github.com/gorilla/mux"
+	"golang.org/x/net/context"
 )
 
 const (