support removing SSH keys entirely

[fgimenezm]

I tried re-opening the existing issue #296 (support removing SSH keys entirely) but I can't so I'm opening this one instead. Feel free to re-open the other one and close this one.

About removing SSH keys:

This is even harder in latest versions. Removing the ssh MachineConfig now tries to remove the "core" user altogether which is not possible and makes the MCO fail to update the nodes.

Originally posted by @fgimenezm in #296 (comment)

[fgimenezm]

This here:
https://github.com/kikisdeliveryservice/machine-config-operator/blob/c415ce6aed25604bc1d2478951db16759dac31f6/pkg/daemon/update.go#L858

why can't it be 0 ?

[kikisdeliveryservice]

reopened old issue, thanks