Add custom root for prepended commands.

I have a project that consists of several parts written in different programming languages(Ruby & Go)​​. Each part performs a specific function.
One part is written in Ruby (RoR) - main. It located in folder `/vagrant/web`. I need to add a prepend_with to vagrant-exec config to start the unicorn server.
```ruby
config.exec.prepend_with 'bundle exec', only: %w(unicorn rails rake)
```
The second part - the worker is written in Go (go-workers). They are located in a different folder (`/vagrant/workers`). For them it is necessary to pass environment variables (GOROOT=\`pwd\`) and do not need to perform `bundle exec`
So I decided to add an extra parameter 'root' to each 'prepend_with' config.