Support Ambari REST API access via the Gateway

Ambari provides a rich REST API feature set. Those APIs should be accessible via the gateway. A few important considerations. Ambari has a different authentication model from the Hadoop cluster do this will need to be dealt with. In addition Ambari isn't part of the Hadoop cluster so the gateway URLs for Ambari should probably peers to the cluster. For example
https://gw-host:gw-port/gateway/clusterA
https://gw-host:gw-port/gateway/ambari
So implementation wise the gateway will likely treat Ambari like a cluster with only the ambari service in it.

Special consideration needs to be taken to understand how authentication for Ambari REST API works and how that can be made secure and performant.