WIP: Feature/caddy frontend slave log expose

Ensure that each slave is correctly exposing front facing and back facing logs with tests. Optionally request frontend from each node to expose logs nicely, instead of hardcoded approach.

Tasks:

• expose and test for frontend-facing log
• expose and test for backend-facing log
• adapt logging done by rsyslogd, as maybe there is needless duplication of timing
• request real slave frontend for nice access will be done in https://lab.nexedi.com/nexedi/slapos/merge_requests/791
• review the way how information is returned to the slave requester
  • it's ok for now
• review log retention
• update documentation no changes needed
• update changes no changes needed