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
* 2.0.
 */
package org.elasticsearch.xpack.security.authc.service;
import org.apache.http.util.EntityUtils;
import org.elasticsearch.client.Request;
import org.elasticsearch.client.RequestOptions;
import org.elasticsearch.client.Response;
import org.elasticsearch.client.ResponseException;
import org.elasticsearch.common.Strings;
import org.elasticsearch.common.bytes.BytesArray;
import org.elasticsearch.common.settings.SecureString;
import org.elasticsearch.common.settings.Settings;
import org.elasticsearch.common.util.concurrent.ThreadContext;
import org.elasticsearch.common.xcontent.XContentHelper;
import org.elasticsearch.core.PathUtils;
import org.elasticsearch.test.cluster.ElasticsearchCluster;
import org.elasticsearch.test.cluster.util.resource.Resource;
import org.elasticsearch.test.rest.ESRestTestCase;
import org.elasticsearch.xcontent.ToXContent;
import org.elasticsearch.xcontent.XContentType;
import org.elasticsearch.xcontent.json.JsonXContent;
import org.elasticsearch.xpack.core.security.authz.store.ReservedRolesStore;
import org.elasticsearch.xpack.core.security.user.KibanaSystemUser;
import org.junit.BeforeClass;
import org.junit.ClassRule;
import java.io.FileNotFoundException;
import java.io.IOException;
import java.net.URISyntaxException;
import java.net.URL;
import java.nio.charset.StandardCharsets;