Nginx: Set server_names_hash_bucket_size to 512 to avoid fatal error.

Without this setting there is fatal error on a fresh install on Debian Wheezy (default 32 is far too low value):

nginx: [emerg] could not build the server_names_hash, you should increase server_names_hash_bucket_size: 32
nginx: configuration file /etc/nginx/nginx.conf test failed