Add monitoring for Juniper devices

Summary

Add monitoring for Juniper devices: QFX, EX, MX and SRX

  1. add juniper_devices struc to values.yaml & values.schema
  2. add junos-exporter
  3. generate the config file required by junos-exporter
  4. add alerting rules
  5. add Grafana dashboard
Edited by Alin H