--edit : I've found the file, it on /root/ispconfig/isp/conf/vhost.conf.master and now it's OK --
I thought you wanted the wildcard option, so that's why I suggested the main apache2.conf file. You're right if you want to set it per website: edit the master template of ISPConfig. Beware that with an upgrade your change gets lost. Please 'vote' for the RFE to add the possibility to create custom templates that will stay, even with upgrades of ISPConfig.