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/fleet/src/javaRestTest/java/org/elasticsearch/xpack/fleet/FleetSystemIndicesIT.java
String responseBody = EntityUtils.toString(response.getEntity());
        assertThat(responseBody, not(containsString("xpack.fleet.template.version"))); 
        assertThat(responseBody, containsString("data"));
        assertThat(responseBody, containsString("bid"));
    }
    public void testCreationOfFleetArtifacts() throws Exception {
        Request request = new Request("PUT", ".fleet-artifacts");
        Response response = client().performRequest(request);
        assertEquals(200, response.getStatusLine().getStatusCode());
        request = new Request("GET", ".fleet-artifacts/_mapping");
        response = client().performRequest(request);
        String responseBody = EntityUtils.toString(response.getEntity());
        assertThat(responseBody, containsString("encryption_algorithm"));
        request = new Request("GET", ".fleet-artifacts-7/_mapping");
        response = client().performRequest(request);
        responseBody = EntityUtils.toString(response.getEntity());
        assertThat(responseBody, containsString("encryption_algorithm"));
    }
    public void testCreationOfFleetEnrollmentApiKeys() throws Exception {
        Request request = new Request("PUT", ".fleet-enrollment-api-keys");
        Response response = client().performRequest(request);
        assertEquals(200, response.getStatusLine().getStatusCode());
        request = new Request("GET", ".fleet-enrollment-api-keys/_mapping");
        response = client().performRequest(request);
        String responseBody = EntityUtils.toString(response.getEntity());
        assertThat(responseBody, containsString("api_key_id"));
        request = new Request("GET", ".fleet-enrollment-api-keys-7/_mapping");
        response = client().performRequest(request);
        responseBody = EntityUtils.toString(response.getEntity());
        assertThat(responseBody, containsString("api_key_id"));