Deployment manager authentication with Cloud Endpoints

I wanted to know more about the deployment manager authentication against a cloud endpoint.
Documentation states that dm oauth authentication with cloud endpoints works , but there are no examples to guide through.

From the documentation

> Related to the previous point, the API has either basic authentication, or, if the API is running on Google Kubernetes Engine or Google Cloud Endpoints, the API supports OAuth 2.0 authentication using an access token from the project's service account.

I have a app engine API service behind an cloud endpoint. If I have to invoke this API using deployment manager, what are the steps that are needed ?

I used the description url for the Google deployment manager to be the rest api link of the app engine service
```
imports:
- path: echo.jinja

resources:
- name: echo
type: echo.jinja
properties:
discoverydoc: https://some-test.appspot.com/_ah/api/discovery/v1/apis/echo/v1/rest
```

```
{% set RC_TYPE_PROVIDER = env['project'] + '/leotaskprovider:echo.get_user_email' %}

resources:
- name: leotaskprovider
type: deploymentmanager.v2beta.typeProvider
properties:
descriptorUrl: {{ properties["discoverydoc"] }}
options:
inputMappings:
- fieldName: Authorization
location: HEADER
value: >
$.concat("Bearer ", $.googleOauth2AccessToken())

- name: get-user-email
action: {{ RC_TYPE_PROVIDER }}
metadata:
dependsOn:
- leotaskprovider

```
I cannot get the deployment manager successfuly authenticate against a oauth enabled cloud endpoint.
Any pointers ?