moving sites-enabled to /etc/nginx/sites-enabled

This is a small change that moves our sites-enabled directory to `/etc/nginx/sites-enabled`.

It does not remove the old directory `/edx/app/nginx/sites-enabled` which will no longer be used, it also keeps the sources in `/edx/app/nginx/sites-available`

fixed, I think what we want to do is just get rid of the nginx configuration diversion altogether. the only difference between the template and the OS package version is the log directory which I think we are ok to move back to /var/log to keep things standard