Make WordPress installation symlink conditional

Some development WordPress installations use Vagrant synced folders to map a WordPress codebase into a VM/container:

```ruby
config.vm.synced_folder ".", "/var/www/html", owner: "www-data", group: "www-data"
```

In these cases, creating a symlink of the codebase into the `wordpress_site_root` is not necessary. To prevent this role from creating a symlink into the web root, set `wordpress_site_env` to `development`.