mantle: replace deprecated golang.org/x/net/context import

SA1019: "golang.org/x/net/context" is deprecated: Use
the standard library context package instead.

Author: prestist

mantle/lang/worker/group.go

@@ -15,10 +15,10 @@
 package worker
 
 import (
+	"context"
 	"sync"
 
 	"github.com/coreos/pkg/multierror"
-	"golang.org/x/net/context"
 )
 
 // Worker is a function that WorkerGroup will run in a new goroutine.

mantle/platform/api/gcloud/image.go

@@ -15,11 +15,11 @@
 package gcloud
 
 import (
+	"context"
 	"fmt"
 	"runtime"
 	"strings"
 
-	"golang.org/x/net/context"
 	"google.golang.org/api/compute/v1"
 )
 

mantle/platform/platform.go

@@ -16,6 +16,7 @@ package platform
 
 import (
 	"bytes"
+	"context"
 	"encoding/json"
 	"fmt"
 	"io"
@@ -30,7 +31,6 @@ import (
 	"github.com/kballard/go-shellquote"
 	"github.com/pkg/errors"
 	"golang.org/x/crypto/ssh"
-	"golang.org/x/net/context"
 
 	"github.com/coreos/coreos-assembler/mantle/platform/conf"
 	"github.com/coreos/coreos-assembler/mantle/util"