cluster alert improvements

Hi @Sarah Terry,

I'm basically replicating internal LM Website checks, but with Devices due to a dev requirement to capture and track specific JSON object values in the response. All of the cluster devices are pointed to the same host/IP/DNS but assigned to different collectors. Each instance in the template is a unique, largely independent web app environment.

Another use case we have includes monitoring specific Windows Services. All monitored services exist in a cluster-like offering and are named the same, but the service name does change based on the version. I elected to use a single Active Discovery template so I'm not constantly updating a datasource just because the delivery teams pushed a new update.

Instance grouping and subsequently defining alert clusters at the instance group-level across devices would be ideal for both of my use cases.