Allow filtering of cookbooks by username

:fork_and_knife: 

https://trello.com/c/bQlLgEFl/180-support-optional-user-parameter-at-api-v1-cookbooks-to-return-all-cookbooks-for-a-given-username


You can do this in one shot with something like:

```ruby
joins(owner: :chef_account).where('accounts.username = ?', username)
```

This has the added bonus that it won't raise a `NoMethodError` if/when `username` does not correspond to a known Chef username.