Fix host verification error at first deploy if repo is not included to the list of known hosts

Fix the issue #90 that I still run into (0.3.1)
I've changed deploy.rb template so ssh-keyscan will be invoked at setup process.

It seems no to be a best solution but it works by now.
If it's controversal to a mina style, I will appreciate any help or advise on how it should be done properly.