Automatically load Google auth

This change removes the hardcoded Google auth mechanism for the GCS
backend, instead using Google's provided client library to discover and
generate credential material.

Google recommend that client libraries use their common auth mechanism
in order to authorise requests against Google services. Doing so means
you automatically support various types of authentication, from the
standard GOOGLE_APPLICATION_CREDENTIALS environment variable to making
use of Google's metadata API if running within Google Container Engine.

internal/backend/gs/config.go

@@ -8,13 +8,13 @@ import (
 	"github.com/restic/restic/internal/options"
 )
 
-// Config contains all configuration necessary to connect to a Google Cloud
-// Storage bucket.
+// Config contains all configuration necessary to connect to a Google Cloud Storage
+// bucket. We use Google's default application credentials to acquire an access token, so
+// we don't require that calling code supply any authentication material here.
 type Config struct {
-	ProjectID   string
-	JSONKeyPath string
-	Bucket      string
-	Prefix      string
+	ProjectID string
+	Bucket    string
+	Prefix    string
 
 	Connections uint `option:"connections" help:"set a limit for the number of concurrent connections (default: 20)"`
 }