Implement first draft for event-handler at device level

Inputs:
-------

```
event_handler:
  evpn-blacklist-recovery:
    action_type: bash
    action: FastCli -p 15 -c “clear bgp evpn host-flap”
    delay: 300
    trigger: on-logging
    regex:  EVPN-3-BLACKLISTED_DUPLICATE_MAC
    asynchronous: true
```

Todo:
-----

- Update documentation templates
- Report to EVPN data model (eos_l3ls_evpn)

Report Envent Handler configuration information into device
auto-generated documentation

Translate group_vars input for event-handler to device data model.

Because data model is the same for both EVPN group_vars and device
data-model, it is important to define event-handler on a per inventory
group based