Use CLI context when running tunnel (#597)

When embedding the tunnel command inside another CLI, it
became difficult to test shutdown behavior due to this leaking
tunnel. By using the command context, we're able to shutdown
gracefully.

cmd/cloudflared/tunnel/cmd.go

@@ -340,7 +340,7 @@ func StartServer(
 	logClientOptions(c, log)
 
 	// this context drives the server, when it's cancelled tunnel and all other components (origins, dns, etc...) should stop
-	ctx, cancel := context.WithCancel(context.Background())
+	ctx, cancel := context.WithCancel(c.Context)
 	defer cancel()
 
 	go waitForSignal(graceShutdownC, log)