Setting the default availability zone dropdown in Horizon

Issue

• When you have more than one availability zone the default to launch an instance in Horizon GUI is "Any availability zone". We would like to have that default dropdown set to "nova" to make any user change it to select something else. We have only found /etc/nova/nova.conf setting default_availability_zone=nova commented out, which we assume would be the default setting, but the GUI does not default to that setting so we're looking for how to change that.