Tie AccessProvider back to service Provider.

To issue a revokation request, I needed to know the Identity API
endpoint.  This endpoint exists in the Provider used to create the
AccessProvider.  This change threads a link back to the Provider.
diff --git a/servers_test.go b/servers_test.go
index 46ba6db..d95fa40 100644
--- a/servers_test.go
+++ b/servers_test.go
@@ -20,6 +20,10 @@
 	return ""
 }
 
+func (ta *testAccess) Revoke(string) error {
+	return nil
+}
+
 func TestGetServersApi(t *testing.T) {
 	c := TestContext().UseCustomClient(&http.Client{Transport: newTransport().WithResponse("Hello")})