Switch from x/net/context -> context

Since Go 1.7, context is a standard package. Since Go 1.9, everything
that is provided by "x/net/context" is a couple of type aliases to
types in "context".

Many vendored packages still use x/net/context, so vendor entry remains
for now.

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

integration-cli/docker_api_inspect_test.go

@@ -1,11 +1,10 @@
 package main
 
 import (
+	"context"
 	"encoding/json"
 	"strings"
 
-	"golang.org/x/net/context"
-
 	"github.com/docker/docker/api/types"
 	"github.com/docker/docker/api/types/versions/v1p20"
 	"github.com/docker/docker/client"