Add default environment variables for clientId and accessToken (similar ...

...to AWS CLI stuff)
Sorry... this is the wrong place to load environment variables...


The correct place is in `var _defaultOptions = {};`, which should just be:
```js
var _defaultOptions = {
  credentials: {
     clientId: process.env.TASKCLUSTER_CLIENT_ID,
     accessToken: process.env.TASKCLUSTER_ACCESS_TOKEN
  }
};
```

Then if you define `credentials` to anything that isn't false, then you won't load default credentials, which be default comes from environment variables... And can be overwritten using `taskcluster.config({...})`.

Ie. `new taskcluster.Queue({credentials: {}})` will give you a `queue` without any credentials. And 
`new taskcluster.Queue({})` or `new taskcluster.Queue({credentials: null})` will give you one with default credentials. Where _default credentials_ is provided by environment variables, if not overwritten by `taskcluster.config({credentials: {...}})`.