I don't really like this pattern... Why not check for the existence of the `test-kitchen` gem? What's the advantage of using an `ENV` variable?

Also, this occurs 3x in the codebase. Can we create a `test_kitchen.rb` class and do all of this in there?

```ruby
# lib/berkshelf/test_kitchen.rb
begin
  require 'kitchen/cli'
rescue LoadError
end

if defined?(Kitchen::Cli)
  class Berkshelf::Cli
    register(Kitchen::CLI, 'test', 'test [COMMAND]', 'Testing tasks for your cookbook')
  end

  # don't know if this is valid
  Berkshelf::InitGenerator.class_options.push(:skip_test_kitchen, type: :boolean, default: false, desc: "Skip adding a testing environment to your cookbook")
end
```

Totally untested, but I think you get what I'm saying.