[WIP] Try using plain vagrant-berkshelf instead of vagrant-toplevel-cookbooks (alternative approach)

[tknerr]

This PR tries to achieve the same as #4, namely replacing the need for vagrant-toplevel-cookbooks by using a plain vagrant-berkshelf solution.

In contrast to #4, the approach here is to use separate Berksfiles per VM rather than a single Berksfile with groups:

• using config.berkshelf.path = "Berksfile.<name>" to define disjoint Berksfiles
• defining one Berksfile per VM

It seems to work quite well in the first experiments. Notes:

• it creates a separate .lock file for each Berksfile -- so there's no potential conflict here as with using the groups feature
• (!) you need to delete the .lock file when updating the cookbook version via the tag ref. rm Berksfile.app_v1.lock && vagrant provision app_v1 works :+1:

[tknerr]

Still, even if that works, there's some reason for vagrant-toplevel-cookbooks though:

1. not cluttering your project with so many Berksfiles
2. no need to remove the .lock files before updating a version
3. programmability from within the Vagrantfile

Last one could be achieved by having a function or "mini-plugin" in the .vagrant_plugins file as well, but that would have to be created per-project. So having a globally installed vagrant plugin makes sense as well.