Don't fire sdw-notify during provisioning

[eloquence]

Follow-up to #445.

#445 adds an hourly cron job that shows a security notice if the system hasn't been updated in a while, or we have no/bad data about when it's been updated. There's no point in showing this warning during the somewhat lengthy provisioning process, and the cron job may fire while provisioning is still underway. Let's add logic to suppress it from firing if we detect that an installation is underway.