cp: Avoid performing DNS queries on every healthcheck execution
This is achieved by targeting the realserver IP or the VIP depending on the forwarding plane:
- Katran: VIP address is used
- IPVS: realserver address is used
We cannot use the VIP with IPVS and IPIP cause we trigger a double encapsulation scenario as described on the phab task
This MR ships commits from !50 (merged), !51 (merged) and !52 (merged)
Closes T372731