Only install oh-my-zsh for users that exist.

Not all items in the `users` data bag may actually be created as users. For example, `recipe[users::sysadmins]` only creates users in the `sysadmins` group, whereas other recipes may use other criteria to decide which users to create under which circumstances.

The recipes in this cookbook should therefore not assume that every item in the `users` data bag has been created as a user on the system. Doing so can cause these recipes to fail when they attempt to add files to the home directories of users that do not exist.