Add instruction for configuring host keys on HA secondary servers
What does this MR do?
Add instruction for configuring host keys on HA secondary servers in HA documentation.
Why was this MR needed?
This wasn't clear in HA docs before and customer's were experiencing false man-in-the-middle-attack alert
What are the relevant issue numbers?
https://gitlab.com/gitlab-org/gitlab-ee/issues/5905
/cc @lbot