Update nginx recipe

- Remove include of SSL certificate recipe.  It isn't configurable, so we'll
  build a custom recipe instead.
- Allow the cookbook containing the nginx.conf template to be overridden, so it
  can be customized.
- Reload nginx after conf file changes.
- Correct path to nginx executable.  It's unclear why the orignal value was
  wrong - maybe something changed in homebrew since this repo was forked.