Add an option to install just Cloud Datastore.

impl/cloud/context.go

Index: impl/cloud/context.go
diff --git a/impl/cloud/context.go b/impl/cloud/context.go
index 527f8e3d149fabf23a4616b61726e3381266288f..001f605955a7aea4f507f30919566c4445985b6a 100644
--- a/impl/cloud/context.go
+++ b/impl/cloud/context.go
@@ -18,9 +18,13 @@ import (
 //
 // This is built around the ability to use cloud datastore.
 func Use(c context.Context, client *datastore.Client) context.Context {
+	return UseDS(useInfo(c), client)
+}
+
+// UseDS installs the cloud datastore implementation into the supplied Context.
+func UseDS(c context.Context, client *datastore.Client) context.Context {
 	cds := cloudDatastore{
 		client: client,
 	}
-
-	return cds.use(useInfo(c))
+	return cds.use(c)
 }