Delete by name if instance_id isn't found

This patch makes it possible to delete openstack servers by name. 

The reasoning here is that we use both vmware and openstack for our infrastructure. With knife-vmware we're able to delete by fqdn, and we'd like to be able to do the same with knife-openstack so that there is consistency in our internal tools. 

I'd love to hear any thoughts from the maintainers on this.