Add monitoring for new scheduler updater 's consumer/writer services

For example, the ghtorrent consumer services needs a persistent opened ssh connection. This is at the moment deployed through a systemd service unit (ssh-ghtorrent.service on saatchi). This should already restart when down (as systemd's dsl permits).

That might now be enough though and some icinga2 alerts could/should be raised if something is off.

Services concerned:

1. ssh-ghtorrent.service: persistent ssh connection with ghtorrent's rabbitmq forward port from remote 5672 to 2765
2. swh-scheduler-updater-consumer-ghtorrent.service: ghtorrent consuming events service (depends on 1.) which writes events in scheduler updater db
3. swh-scheduler-updater-writer.service: Reads the scheduler updater db and writes the events as oneshot tasks in the scheduler db

---

Migrated from T1078 (view on Phabricator)