Postpone CM configuration change monitoring until the Knox GW is up&running

As of now, Knox starts CM configuration change monitoring right away it starts the DefaultClusterConfigurationMonitorService. This action will trigger the PollingConfigurationAnalyzer even when descriptors with possible service discovery settings are not even initialized.

My suggestion is to take advantage of the recently introduced GatewayStatusService and set the isActive flag to true based on the result of org.apache.knox.gateway.services.topology.impl.GatewayStatusService.status().