Add autostart as an option for vagrant boxes [WIP]

When boxes are defined in the vagrantfile, the "vagrant up" command without arguments always selects all boxes.

With this change, there is an option to add "autostart: false" to a box, in which case it wil only start if specifically used as an argument to vagrant up.

The purpose of this is to allow boxes in the configuration that are used for a specific purpose, but are not needed for regular developers to start.

- Added specs for the environment and vagrantfile.
- Added a methd to retrieve autostart_machine_names.
- Changed the plugin up/command to use autostart_machine_names when no argument was given to the command.

### warning
I am not a Ruby developer, so I'll need some help with this:
The tests pass, including the new specs I added, but I can't seem to run vagrant with "bundle exec vagrant" without getting an error. I don't know if the code actually works.