Why I did it
In upgrade scenarios, where config_db.json is not carry forwarded to new image, it could be left w/o TACACS credentials.
Added a service to trigger 5 minutes after boot and restore TACACS, if /etc/sonic/old_config/tacacs.json is present.
How I did it
By adding a service, that would fire 5 mins after boot.
This service apply tacacs if available.
How to verify it
Upgrade and watch status of tacacs.timer & tacacs.service
You may create /etc/sonic/old_config/tacacs.json, with updated credentials
(before 5mins after boot) and see that appears in config & persisted too.
