Only write minion key to disk if you don't already have it

I've noticed on our salt-masters that there is a LOT of disk activity writing keys to disk if you have auto_accept on-- this is because there is no checking if we already have it or not

As far as I can tell this won't break anything-- and the unit tests don't break (any more than they are currently on develop)
Merge pull request #6795 from jacksontj/develop

Only write minion key to disk if you don't already have it