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

...to AWS CLI stuff)
Okay, so first off... If you wanted to do this we should do:
```
var credentials = _.defaults({}, options.credentials || {}, {
   clientId: process.env.TASKCLUSTER_CLIENT_ID,
   accessToken: process.env.TASKCLUSTER_ACCESS_TOKEN
});
```

But there are API end-points that don't require authentication. So there are probably cases were you want to be able to create a client without credentials. Maybe we should not support this case... Or support it using `{credentials: false}` as a special indicator that this is desired behaviour... I guess it might be a corner case anyways.