Support loading of existing projects

I tested deploy_drupal::default recipe with the use case of loading an installing an existing drupal project (codebase and SQL dump), and needed to make a few fixes in the process to recipes/default.rb.

Additionally I created two new tests in `test/new-drupal-site/Vagrantfile` and `test/existing-drupal-site/Vagrantfile' which establish that the deploy_drupal successfully installs in both of these cases.

Upon merging this branch to master, it will be necessary to modify `test/new-drupal-site/Cheffile` and `test/existing-drupal-site/Cheffile` to remove `:ref => "origin/AD-Existing"` as appropriate.