let plugins block.
- Move engine to Action Executor
- Accept resource_id in the callback url to override contract data. ( for external alarms tenant based)
- Add a notification listener (tracker) able to react to notifications and trigger the
  alarm engine
- A new alarm, NOTIFICATION_ALARM ( react upon notifications ):

    [ Evacuate on each unpause action ( not possible, just an example) ]
   ex: sla-contract-create -tenant_id c8d29398c19846b2b7b7175a8acf73c9 NOTIFICATION_ALARM 2 evacuate
       -resource_id 222  -alarm_data '{"meter": "compute.instance.unpause.end"}'

- Add block to evacuate , won't return till finish. ( ugly ). may improve when task land in trunk
A new service is required, this time run
        python healing/cmd/launch.py --server tracker --config-file etc/healing.conf.example