Adds a rails_path variable

Allows the users to set a `rails_path` variable in their deploy files if the app is nested in a subdirectory of the repo.

I needed to deploy a Rails engine's dummy app so I can stage the UI elements. I added this to deploy the entire engine, but run all Rails specific tasks in `spec/dummy` where the app lives.

I know it's a pretty specific use case, but there could be others, so I thought I'd suggest the feature.

thanks


How about using `File.join`?
I think `fetch` takes a second parameter, which is used as default value if empty.