Add Support for Choosing a VPC

[utkonos]

The concept of a "private network" has been changed to VPCs. Only a new droplet can be added to a VPC. An existing droplet cannot be moved from one VPC to another. Therefore, if you are using VPCs, this Vagrant plugin doesn't let you assign a VPC. Adding an option to specify a VPC as is available in the API would be great. However, I have a workaround that I can recommend until the plugin supports this feature. One VPC at a time can be designnated as the default VPC for a region. If you want a droplet built with Vagrant to be assigned to a particular VPC, make that one the default for the region temporarily during vagrant up. Once the droplet is built, configured, and you can connect to it etc, change the default VPC back to the one you actually want as the default.