JCLOUDS-670 Option force no password when launching images on providers

Adds the option to override whatever password is given from the provider for the image.

See [jira](https://issues.apache.org/jira/browse/JCLOUDS-670)
Also, is it OK to *don't* set the private key if the password is present but empty? I mean, if the private key is actually present, shouldn't we set it, regardless of the password? IIRC the authentication will fallback to the SSH agent if the given key is not the righr one.