Updated vagrant external plugin

This plugin show now:
- Properly support --host and --list arguments appropriately
- Support non standard SSH ports
- Support for vagrant multi-machine

If you are looking for a rig to test this on, you can use my [vagrant_ansible_example](https://github.com/markmandel/vagrant_ansible_example) repo, which I used for testing this plugin.

Please let me know if there are any further issues.