Improve NGINX worker, connections, and body size

* Parameterize NGINX worker_processes to automatically reflect the
  number of CPU cores in the system.

* Set higher worker_connections, which should be fine given the
  number of allowed open file descriptors.

* Update some comments about client_max_body_size.  Explicitly
  declare this in the Wordpress NGINX virtual host, and make it
  consistent between blocks that relate to the frontend site.

* Also increase PHP / Wordpress max. request body size, which
  affects the maximum size of a file uploaded to the Wordpress
  site.
Merge pull request #34 from dpla/nginx-workers-connections

Improve NGINX worker, connections, and body size