credentialsSecret is a required field, but also has the empty string as a default value

[fhennig]

While working on something else I noticed that the credentialsSecret field in the clusterConfig is not optional (i.e. it is required), but we also derive Default for the whole struct, so the field gets a default value, which is the empty string for the string type.

This seems like a bug to me.

Not urgent.