Configurable scope for Dashboard Clients

* Dashboard clients are given scope of cc_service_broker_client but no
* more than openid, cloud_controller.read, cloud_controller.write, and
* cloud_controller_service_permissions.read.

[#71594888]
Merge pull request #252 from cloudfoundry/feature/71594888-dashboard-client-scope

Configurable scope for Dashboard Clients
Update README to refer to rake task

Signed-off-by: Zach Robinson <zrobinson@pivotallabs.com>