Add BlueboxServerCreate#server for external tooling.

This would provide a mechanism for other Knife plugins to use
BlueboxServerCreate and get a reference to the newly created server.
Useful activities may include obtaining the IP address, hostname,
block_id, etc.

The driving use case is to add Blue Box Blocks support to the
[knife-server](http://fnichol.github.io/knife-server/) gem.