consider de(bi|vu)an version

cdist/conf/type/__grafana_dashboard/manifest

@@ -1,7 +1,10 @@
 os=$(cat $__global/explorer/os)
+os_version=$(cat $__global/explorer/os_version)
 
 case $os in
     debian|devuan)
+        case $os_version in
+            8*|jessie)
                 __apt_key_uri grafana \
                    --name 'Grafana Release Signing Key' \
                    --uri https://packagecloud.io/gpg.key
@@ -16,6 +19,12 @@ case $os in
                 require="__apt_source/grafana __package/apt-transport-https" __package grafana
                 require="__package/grafana" __start_on_boot grafana-server
             ;;
+            *)
+                echo "Don't know how to install Grafana on $os $os_version. Send us a pull request!"
+                exit 1
+            ;;
+        esac
+    ;;
     *)
         echo "Don't know how to install Grafana on $os. Send us a pull request!"
         exit 1