fix provisioning for host provider by passing private key to Net::SCP.st...

Provisioning with the host provider was broken, because the private key was never passed to [`Net::SCP.start()`](http://net-ssh.github.com/scp/v1/api/classes/Net/SCP.html) and thus authentication failed.

With this PR:

 * `user`, `private_key_path` and `port` as defined in `Mccloudfile` are used by default
 * but can be overridden by explicitly passing the `options` parameter (I guess this was the intention)