[refactor] Poll SSH instead of fixed timeout while bootstrapping

Various compute providers need different time to bring the VM up and
make it accessible under some IP. Previously we'd simply wait a minute
before trying to bootstrap the server.
This polls VM over SSH to determine when it's up and ready to accept
commands.
Merge pull request #97 from nodejitsu/ssh-poll

[refactor] Poll SSH instead of fixed timeout while bootstrapping