Part of https://github.com/elastic/kibana/issues/154715

The `.fleet-secrets` system index should just have a value key. 

Kibana system should only have write and delete access NOT read access.

Fleet server will have read access only. 

Example document:

```
// example document from the .fleet-secrets index 
{
	_id: 'abcd1234'
	_source: {
	   value: "abcd12345678",
      }
}
```
 
x-pack/plugin/security/qa/service-account/src/javaRestTest/java/org/elasticsearch/xpack/security/authc/service/ServiceAccountIT.java
"write",
                    "monitor",
                    "create_index",
                    "auto_configure",
                    "maintenance"
                  ],
                  "allow_restricted_indices": true
                },
                {
                  "names": [
                    "synthetics-*"
                  ],
                  "privileges": [
                    "read",
                    "write",
                    "create_index",
                    "auto_configure"
                  ],
                  "allow_restricted_indices": false
                }
              ],
              "applications": [        {
                  "application" : "kibana-*",
                  "privileges" : [
                    "reserved_fleet-setup"
                  ],
                  "resources" : [
                    "*"
                  ]
                }      ],