Pass a :keys option to Net::SSH

For the --private-key option to work, we need to consistently
use the :keys option when accessing remote servers.
Make SSH keys set with --private-key work in 'bosh-bootstrap ssh'

The code to do this was already present, but commented out.  The
only reason it didn't work is because "-i #{private_key_path}" makes
ssh attempt to use the key path with a prepended space char.