File.exists? calls within init generator must include the destination root for portability purposes

Any calls to things on the file system must be made relative to the destination root of the Thor group.

This can be done by `File.join`'ing on `destination_root` from within the generator.

https://github.com/opscode/test-kitchen/blob/master/lib/kitchen/generator/init.rb#L76

Without this, the generator could fail if a different destination root is given. Example:
```ruby
Kitchen::Generator::Init.new(Array.new, Hash.new, destination_root: "/some/where").invoke_all
```