Prevent ATS v2 health check REST API call if the ATS service itself is disabled.

Currently, even if yarn.timeline-service.enabled is disabled, the UI2 code still goes ahead and performs timeline health check REST API calls, see [1], [2] and [3].

This is un-necessary and can cause slowness issues with RM UI2 page loading if a firewall is dropping packets on the ATS v2 port (as it is not meant to be available e.g. 8188/8190) in the background and the timeout is not yet hit.

This ATSv2 health check REST API call is redundant and should be prevented if the service itself is disabled.

[1] https://github.com/apache/hadoop/blob/trunk/hadoop-yarn-project/hadoop-yarn/hadoop-yarn-ui/src/main/webapp/app/adapters/timeline-health.js
[2] https://github.com/apache/hadoop/blob/trunk/hadoop-yarn-project/hadoop-yarn/hadoop-yarn-ui/src/main/webapp/app/routes/application.js#L34
[3] https://github.com/apache/hadoop/blob/trunk/hadoop-yarn-project/hadoop-yarn/hadoop-yarn-ui/src/main/webapp/app/routes/yarn-app/logs.js#L40