Add Juniper devices monitoring

Add Juniper devices monitoring support

sylva-junos-exporter unit is similar to sylva-snmp-exporter and uses a helper unit to generate its config file sylva-junos-config

juniper: struct defines the devices that will be monitored

• Each device can use its own authentication method that should be present in juniper.auth
• Several devices can be grouped (by specifying the same device_group and redundancy_group ) as some alerting rules will trigger only if the majority or all members of a group experience the same problem.
• The default features list that applies to all devices can also be (partially) overwritten in juniper.features and each device can enable/disable addtional featues
• The job's scrape interval and timeout can be controlled by monitoring.prometheus.jobs.junos_exporter.scrape_interval and monitoring.prometheus.jobs.junos_exporter.scrape_timeout for devices that take longer to scrape
• SSH key based auth is possible and the junos-exporter chart supports specifying the private key (base64 encoded representation.. cat $HOME/.ssh/id_rsa | base64 -w0 && echo) and will mount it in a specified path
• Preflight check for a device referencing a non-existing auth
• Validation also handles the case when the user configured auth, but no devices. It would not enable the units, but might have made it difficult to determine why
• Add annotation of juniper so we trigger a redeploy of the exporter on config change

Closes: #3443 (closed)