Make WordPress installation symlink conditional

Some development WordPress installations use Vagrant synced folders to
map a WordPress codebase into a VM/container:

  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`.