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/core/src/main/java/org/elasticsearch/xpack/core/security/authz/store/ReservedRolesStore.java
}
    private static Map<String, RoleDescriptor> initializeReservedRoles() {
        return MapBuilder.<String, RoleDescriptor>newMapBuilder()
            .put("superuser", SUPERUSER_ROLE_DESCRIPTOR)
            .put(
                "transport_client",
                new RoleDescriptor(
                    "transport_client",
                    new String[] { "transport_client" },
                    null,
                    null,
                    MetadataUtils.DEFAULT_RESERVED_METADATA
                )
            )
            .put("kibana_admin", kibanaAdminUser("kibana_admin", MetadataUtils.DEFAULT_RESERVED_METADATA))
            .put(
                "kibana_user",
                kibanaAdminUser("kibana_user", MetadataUtils.getDeprecatedReservedMetadata("Please use the [kibana_admin] role instead"))
            )
            .put(
                "monitoring_user",
                new RoleDescriptor(
                    "monitoring_user",
                    new String[] { "cluster:monitor/main", "cluster:monitor/xpack/info", RemoteInfoAction.NAME },
                    new RoleDescriptor.IndicesPrivileges[] {
                        RoleDescriptor.IndicesPrivileges.builder()
                            .indices(".monitoring-*")
                            .privileges("read", "read_cross_cluster")
                            .build(),
                        RoleDescriptor.IndicesPrivileges.builder()