Use systemd notification where applicable

With this change in place, the grafana service will signal
readiness to serve by writing "READY=1" to the path specified
through the NOTIFY_SOCKET environment variable. If this
environment variable is not present or empty, no notification
will happen. This notification is the standard systemd
mechanism for indicating a service is ready to serve. For
Grafana this may be a couple of seconds from startup due to
database migrations. This change also adjusts the Grafana
systemd service definition to make use of this feature.

pkg/cmd/grafana-server/server.go

@@ -29,6 +29,7 @@ import (
 
 	"github.com/grafana/grafana/pkg/social"
 	"github.com/grafana/grafana/pkg/tracing"
+	"github.com/grafana/grafana/pkg/util"
 )
 
 func NewGrafanaServer() models.GrafanaServer {
@@ -96,6 +97,7 @@ func (g *GrafanaServerImpl) Start() {
 		return
 	}
 
+	util.SdNotify("READY=1")
 	g.startHttpServer()
 }