re-install cluster node

My guess on /etc/openvswitch is yes at least make a backup so you can use it later if needed.
About /etc/ssh, unless you changed something yourself you can leave those.

I think both come from a default package install but you never know if someone touched them afterwards.
 
Then indeed back them up! Strange this is not in the documentation. I just checked, I have

ssh_host_dsa_key
ssh_host_dsa_key.pub
ssh_host_ecdsa_key
ssh_host_ecdsa_key.pub
ssh_host_rsa_key
ssh_host_rsa_key.pub
ssh_known_hosts -> /etc/pve/priv/known_hosts

They were created when I created the cluster. Might be that corosync syncs them when cluster gets a quorum?
 

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