Add autostart as an option for vagrant boxes

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.