reinstalling k3s without changing the hostname

k3s#467

When an agent joins a k3s cluster, the agent generates a password that the server stores for future authentication requests, based on the hostname. Reinstalling k3s to that agent will throw 403s as the newly generated password obviously won’t match what the server has stored from the previous installation.

Delete the relevant password stored in /var/lib/rancher/k3s/server/cred/passwd so that the new agent installation can successfully hand the server the newly generated password.