Vagrant file for launching manager/worker machines described in Docker swarm mode tutorial

This file is to launch the following VMs described in https://docs.docker.com/engine/swarm/swarm-tutorial/ by running vagrant up. You can test and evaluate docker swarm mode which has been introduced since Docker 1.12.

1. manager1
   • 192.168.99.100
   • vagrant ssh manager1
2. worker1
   • 192.168.99.101
   • vagrant ssh worker1
3. worker2
   • 192.168.99.102
   • vagrant ssh worker2

All the above machines will run initialization commands on its beginning to install docker >= 1.12.