feat: Add self-defined template configure file for embedded flannel

-Add an additional parameter to k3s agent like '--flannel-conf-tpl' to set a template of flannel conf.
-Use that to render a real conf for embedded flannel.
Issue k3s-io#1531

pkg/agent/config/config.go

@@ -450,7 +450,7 @@ func get(envInfo *cmds.Agent) (*config.Node, error) {
 			nodeConfig.FlannelConfOverride = true
 		}
 
-		if nodeConfig.FlannelConfTpl != "" {
+		if envInfo.FlannelConfTpl != "" {
 			nodeConfig.FlannelConfTpl = envInfo.FlannelConfTpl
 			nodeConfig.FlannelConfOverride = true
 		}