Added 'destroy-instance' command to mark AWS instance for termination

Currently, you can use 'modify-instance' to mark the Amazon instance for termination (rename to instance-terminate).
However, this will not remove the Vagrant machine under `.vagrant/machines/default/aws` directory. So if you do the `vagrant up` afterward, then Vagrant will try to re-use the instance that is marked for termination instead of launching a new AWS instance.

This PR will add `destroy-instance` command to Vagrant, that will:

* Rename the AWS instance associated with the current Vagrant machine to `NAME-terminate`
* Cleanup the `.vagrant/machines/default/aws` folder so the `vagrant up` will spawn a new machine