Replicate user behaviour in the probe

[enolfc]

Short Description of the issue

Current probe relies on the https://github.com/EGI-Federation/egi-notebooks-monitoring being deployed alongside the hub to get the status of the hub and its capability to start a user notebook. While this makes the probe implementation really simple, it does need to have permanent services running at the hub side and the monitoring is done with a fake user which may cause false positives/negatives. It would be better to implement the probe at the nagios side, simplifying the service deployment and providing a more realistic status of the service.