Support demo server deployments

User story: I am a Liberty developer. I want to deploy a server application to a DigitalOcean droplet(s), so I can demo it.

Assumptions: The user can manually create a DigitalOcean droplet and ssh onto it.

Out of scope: The demo does not need production-quality stability, security etc.

Suggested solution:

1. Rename the current shipyard tool to vm. So shipwright shipyard create box becomes shipwright vm create box

2. Add a new droplet tool that accepts the same parameters as vm (create, ssh, playbook), but manages a droplet using vagrant-digitalocean

3. Ensure that both of these sequences function as expected:

shipwright vm create my_vm
shipwright vm playbook my_vm ldh_developer/shop.yml

shipwright droplet create my_droplet
shipwright droplet playbook my_droplet ldh_developer/shop.yml