Adapt longhorn storageclass replicas to the number of nodes

Summary

This is an outcome of this discussion: !2928 (comment 2139647772)

Currently, when longhorn is enabled, we are using a default storageclass with 3 replicas and a storageclass with 2 replicas for minio. This can't fit into single node deployments. We should consider to decrease these replicas on single-node-deployments.