Add nodes dynamically to an AppScale deployment

Receives requests from the appscale-tools and dynamically adds nodes to an already running AppScale deployment. Relies on appscale-add-instances (https://github.com/AppScale/appscale-tools/pull/52).

To test, try following the documentation (https://github.com/AppScale/appscale/wiki/Adding-Machines-to-an-AppScale-Deployment) and verifying that it works. That document adds two nodes to an existing four node deployment in VirtualBox.