install cookbooks from Berksfile if present

init generator will generate berkshelf files
Unify Librarian and Berkshelf methods and tests for Cook

- Remove hard dependency on Berkshelf gem
- Add `--no-berkshelf` option to disable cookbook installation
Unify Librarian and Berkshelf methods for Init
Add expand_path helper

Ensure that add_cookbook_path parameter is Pathname and expanded.
Allow Berkshelf path to be configured

Default to temporary directory.

TODO:

  - Tests
  - Remove tmpdir after syncing
  - Documentation
Include `:berkshelf` option to SoloBootstrap