(0.9) slight change to user module

I noticed the user module took a "ssh_key=generate". As this was
really acting as a boolean, I've fixed this.

It now takes:

generate_ssh_key=yes|no

This was new for 0.9, so technically, it's still fair game.

Please let me know when you see other things in the module space that
act like booleans but aren't... we want to fix those.

--Michael