Continuously destroy VM until it goes away

This pull request fixes issue #96 by ensuring the VM is gone before trying to do any IP or volume destruction. It uses the Vagrant `Vagrant::Util::Retryable` functionality as used in [`create_server.rb`](https://github.com/cloudbau/vagrant-openstack-plugin/blob/master/lib/vagrant-openstack-plugin/action/create_server.rb#L103) and `server.reload` to await a `Fog::Compute::OpenStack::NotFound` exception. The retyring continues the VM destroy every 10 seconds for 20 attempts until the VM is found dead (polling once a second) or we timeout.

I did try the Fog [`wait_for`](https://github.com/fog/fog/blob/master/lib/fog/openstack/docs/compute.md#model-layer) method to poll for the server going away. However, it never seemed to return.

This pull request depends on #95 being merged first.