provider/google: Allow acctests to set credentials via file

Makes things easier on Travis.

builtin/providers/google/provider_test.go

@@ -1,6 +1,7 @@
 package google
 
 import (
+	"io/ioutil"
 	"os"
 	"testing"
 
@@ -29,6 +30,14 @@ func TestProvider_impl(t *testing.T) {
 }
 
 func testAccPreCheck(t *testing.T) {
+	if v := os.Getenv("GOOGLE_CREDENTIALS_FILE"); v != "" {
+		creds, err := ioutil.ReadFile(v)
+		if err != nil {
+			t.Fatalf("Error reading GOOGLE_CREDENTIALS_FILE path: %s", err)
+		}
+		os.Setenv("GOOGLE_CREDENTIALS", string(creds))
+	}
+
 	if v := os.Getenv("GOOGLE_CREDENTIALS"); v == "" {
 		t.Fatal("GOOGLE_CREDENTIALS must be set for acceptance tests")
 	}