makes datasource handshake more explicit

pkg/plugins/datasource_plugin.go

@@ -62,8 +62,8 @@ func (p *DataSourcePlugin) Load(decoder *json.Decoder, pluginDir string) error {
 
 var handshakeConfig = plugin.HandshakeConfig{
 	ProtocolVersion:  1,
-	MagicCookieKey:   "GRAFANA_BACKEND_DATASOURCE",
-	MagicCookieValue: "55d2200a-6492-493a-9353-73b728d468aa",
+	MagicCookieKey:   "grafana_plugin_type",
+	MagicCookieValue: "datasource",
 }
 
 func buildExecutablePath(pluginDir, executable, os, arch string) string {