[DOC] Add notes on disabling permissions for Data Processing

In order to use Sahara from Horizon in the case where Sahara is
not registered with keystone, the service-based permissions on
the Sahara panels must be removed.

Change-Id: I5936ae6862c24ce8bfe7b55318234c1f4bae7925
Closes-bug: 1379906
Merge "[DOC] Add notes on disabling permissions for Data Processing"