I am disabling ceilometer on my OpenStack Ocata cloud.

As fas as I understand, besides stopping the ceilometer services, I can also apply the following changes to nova.conf on the compute nodes:  

instance_usage_audit = True --> false
notify_on_state_change = vm_and_task_state --> None

I have a problem with the latter change:

but in the nova log:

2019-01-21 08:31:48.246 6349 ERROR nova ConfigFileValueError: Value for option notify_on_state_change is not valid: Valid values are [None, vm_state, vm_and_task_state], but found 'None'

I have also tried setting 

notify_on_state_change=<None>

but it complains that it is not a valid value

I can simply comment that line, but I am afraid there is a problem somewhere

Thanks, Massimo