Add glide.yaml and vendor deps

vendor/github.com/docker/distribution/registry/storage/util.go

@@ -0,0 +1,21 @@
+package storage
+
+import (
+	"github.com/docker/distribution/context"
+	"github.com/docker/distribution/registry/storage/driver"
+)
+
+// Exists provides a utility method to test whether or not a path exists in
+// the given driver.
+func exists(ctx context.Context, drv driver.StorageDriver, path string) (bool, error) {
+	if _, err := drv.Stat(ctx, path); err != nil {
+		switch err := err.(type) {
+		case driver.PathNotFoundError:
+			return false, nil
+		default:
+			return false, err
+		}
+	}
+
+	return true, nil
+}