Be more flexible when comparing node roles labels for PVC reuse

[sebgl]

We currently require a strict node role mapping while attempting to reuse PVCs.

This helps us make sure a master node does not get assigned a data volume, and the other way around.

However consider the following case:

• 3 master-data nodes
• the user enables the ML role on all nodes

Currently we'll not reuse PVCs for those 3 ML nodes, and go through data migration. But we could reuse PVCs. Should we also reuse PVCs the other way around (data-ML become data only)?

[sebgl]

We decided to rely on one StatefulSet per NodeSpec in the Elasticsearch CRD.
PVC reuse is now limited to NodeSpecs with the same name. It's basically up to the user to control PVC reuse: keep the nodeSpec name if you want to reuse PVCs, use another name if you want fresh new volumes.