Role fails if `nd_alarm_slack_url` isn't defined
https://gitlab.cri.epita.fr/cri/iac/ansible/roles/netdata/-/blob/0.1.0/templates/health_alarm_notify.conf#L186 causes template generation to fail if no Slack webhook is configured.
https://gitlab.cri.epita.fr/cri/iac/ansible/roles/netdata/-/blob/0.1.0/templates/health_alarm_notify.conf#L186 causes template generation to fail if no Slack webhook is configured.