Add vagrant-cachier support to default Vagrantfile.erb

[tfitch]

I know that there is now the ability to use a custom Vagrantfile template or inject arbitrary code in to the Vagrantfile, but I'm still interested in adding this addition because it would be reverse compatible (because of the if + has_plugin? combo) for those that don't use the vagrant-cachier plugin. https://github.com/fgrehm/vagrant-cachier

So as a user I want support for the vagrant-cachier plugin if I have it installed on my machine by adding these lines to the default Vagrantfile.erb template.

  if Vagrant.has_plugin?("vagrant-cachier")
    c.cache.scope = :box
    c.cache.auto_detect = true
  end

But it's not that simple to implement well because scope and auto_detect have a couple of value options, so we'd want them to be configurable and not force people to use just that combination of values. That said, we need to draw a line somewhere and this is a great basic implementation. If the user wants to dig in to advanced vagrant-cachier settings like synched_folder_opts then they need to go in to Test Kitchen power user mode with a custom Vagrantfile.erb or inject the custom settings.

[tknerr]

@tfitch I have this in my global ~/.vagrant.d/Vagrantfile. This kicks in for every test kitchen:
https://github.com/tknerr/bills-kitchen/blob/master/files/home/.vagrant.d/Vagrantfile#L9-L27

[cheeseplus]

I'm all for supporting a feature/config option that makes this easy but I also don't want to make it default. The one scenario I am envisioning is if someone has the plugin installed but doesn't intend or want to use it. I think a configuration option makes more sense - that or abstracting this to a plugin.

[cheeseplus]

Also - #200 where I've made the same request to make this toggleable to prevent it from even getting into the Vagrantfile.