Running on multiple hosts concurrently

As discussed in #433, you can run into issues with cookbook vendoring if you try to run knife-solo in parallel with itself.

To work around this currently, add this to your local knife.rb:

knife[:berkshelf_path] = ENV.fetch('BERKSHELF_VENDOR_PATH', "cookbooks")

Then invoke each knife-solo instance with it's own path like so:

$ BERKSHELF_VENDOR_PATH=cb1 knife solo cook ...