Work around an unfortunate limitation of symlink.

In Capistrano 3.0, the symlink tasks only run on :app servers,
meaning important files like database.yml don't get symlinked into
the release on non-:app servers like :delayed_job machines.

Assuming this fixed, there is a further problem that Capistrano 3.0
strictly checks that every symlinked file must exist on every server.
So if for security reasons we don't want to create database.yml on
the :web server, Capistrano will fail the deployment.

This commit address these problems with two workarounds:

1. Create config files like unicorn.rb and database.yml on :all
   servers. This is necessary to pass Capistrano's rigorous :check
   tasks.

2. Introduce our own symlink tasks that run after Capistrano's,
   creating symlinks for servers other than :app.
Ask questions before parallelizing.

Otherwise questions will be asked multiple times as the commands
run on multiple servers simultaneously.