If SSL is going to be forces is better not to use varnish as it does not support this kind of connection well. So the best thing to do is to disable varnish and make Apache listen in port 80.
To disable Varnish, first stop it
sudo /opt/bitnami/ctlscript.sh stop varnish
and then disable it
sudo mv /opt/bitnami/varnish/scripts/ctl.sh /opt/bitnami/varnish/scripts/ctl.sh.disabled
Then change the Apache HTTP port from 81 to 80 using the instructions in the following guide:
And restart Apache
sudo /opt/bitnami/ctlscript.sh restart apache