Every provisioner that uses communicator implements its own retryFunc.
Take the remote-exec implementation (since it's the most complete) and
put it in the communicator package for each provisioner to use.
Add a public interface `communicator.Fatal`, which can wrap an error to
indicate a fatal error that should not be retried.
This is needed as preperation for adding WinRM support. There is still
one error in the tests which needs another look, but other than that it
seems like were now ready to start working on the WinRM part…
James Bardin