Implement Monitoring_AlerthistogramController::hostsFromGroup()

refs #4474
Refactor Monitoring_AlerthistogramController::hostsFromGroup() -> ...::groupMembers()

refs #4474
Make use of Monitoring_AlerthistogramController::groupMembers()

refs #4474
Make Monitoring_AlerthistogramController::createHistogram() can handle any number of host/service filters

refs #4474
Drop Monitoring_AlerthistogramController::getHost() and ...::getService()

refs #4474
Render multiple charts (as needed)

refs #4474