cmd: move agent running message to logs

Previously this line was mixed up with logging, which made the output
quite ugly. Use the logger to output this message, instead of printing
directly to stdout.

This has the advantage that the message will be visible when json logs
are enabled.

command/agent/agent.go

@@ -270,7 +270,7 @@ func (c *cmd) run(args []string) int {
 	// Let the agent know we've finished registration
 	agent.StartSync()
 
-	cli.output("Consul agent running!")
+	c.logger.Info("Consul agent running!")
 
 	// wait for signal
 	signalCh = make(chan os.Signal, 10)