Dear hshafi,
That file looks OK. However, you have not answered any of the questions regarding if the process is running "nginx: master process /usr/sbin/nginx -c /var/lib/zentyal/conf/nginx.conf", or if there are any errors reported in /var/log/zentyal/zentyal.log (you could also check /var/log/zentyal/error.log.
Perhaps you could check to see if nginx is listening on the correct ports by running 'sudo lsof |grep 8443' (without the '), you should get an output similar to: -
nginx 13303 root 9u IPv4 91944 0t0 TCP *:8443 (LISTEN)
nginx 13311 www-data 9u IPv4 91944 0t0 TCP *:8443 (LISTEN)
nginx 13312 www-data 9u IPv4 91944 0t0 TCP *:8443 (LISTEN)
nginx 13313 www-data 9u IPv4 91944 0t0 TCP *:8443 (LISTEN)
nginx 13314 www-data 9u IPv4 91944 0t0 TCP *:8443 (LISTEN)
If you do get these outputs then you should be able to get to the webadmin pages.
If you get no output, then nginx is not running. Check with 'sudo lsof' just to make sure that there are processes listening on your system
I hope this is of use to you. Please give some extra detail with your next response as it is hard to diagnose your issue without more information.