When users do ssh to a host that's not listed in their own
~/.ssh/known_hosts ssh warns them about it being "new" and if the user
wants to add the key to their personal list. This is because
StrictHostKeyChecking is set to yes (the default). The problem is that
this is a computing cluster, and this asking makes a mess to the
users.

The keys are all listed in /etc/ssh/ssh_known_hosts. I'd like that ssh
considers them known and doesn't warn the user nor asks to copy them
to their personal list. How can this be done?

I think StrictHostKeyChecking should not trigger when the keys are in
the system list. I cannot set it to "no" because it should apply to
other hosts outside the site.