Improve Sahara configuration

* Correct Sahara service name in keystone catalog;
* Remove "SAHARA_URL" from dashboard config;
* Remove plugins paths;

This change is for Fuel 5.0 (IceHouse)

Change-Id: I094e8cb52ba7e94c12ae5987f26dd7b9b9b39aef
Merge "Improve Sahara configuration"