Add Grafana support to display performance data

Following steps need to be added/documented
* add config options
  * grafan url
  * grafana api key
  * grafana image_size
  * grafana timeperiod
* read icinga host/service vars (or add config option to define)
  * vars.grafana_dashboard
  * vars.grafana_panelid

Add action button to detailed view and deliver image as attachment