add "activate $HOST" command to my-swap-master

Right now, my-swap-master uses the "swap" command, but my-swap-slave uses the "activate $HOST" command. The latter has proven itself to be less prone to user errors (eg, user aborts, hits UP-ENTER, and accidentally swaps the VIP right back where it started!)

There should be an "activate" command in both tools, with the same behaviour.