roles/nginx: Tweaks for vhosts with WordPress

My WordPress blogs have a /wordpress subdirectory in the document
root, but I don't serve from the /wordpress URI.

Technically, all we need is the tweaks to the try_files:
    - `?args` passes query strings to php5-fpm
    - removing 404 from the vhost's try_files so we don't return 404
    when the requested file doesn't exist (obviously not all request
    URI's in WordPress are actual files on the disk)

Signed-off-by: Alan Orth <alan.orth@gmail.com>