overwrite SCRIPT_FILENAME variable in nginx conf

SCRIPT_FILENAME is defined in /etc/nginx/fastcgi_params . For owncloud
we need to overwrite it, so php gets only the path/filename of the file
to be executed.
Also turn off access_log and 404 logging for robots.txt
Merge pull request #104 from Danez/master

overwrite SCRIPT_FILENAME variable in nginx conf