Add color map Monitoring_AlerthistogramController::$colors

refs #4474
Deduplicate usage of HistogramGridChart::drawLines() by iterating over Monitoring_AlerthistogramController::$colors

refs #4474
Add label map Monitoring_AlerthistogramController::$labels

refs #4474
Unify Monitoring_AlerthistogramController::createHostHistogram() and ...::createServiceHistogram() to ...::createHistogram()

refs #4474