Add Pod Disruption Budget for Stateful Set

[thriqon]

What this PR does / why we need it:

OpenLDAP clusters with multiple replicas should ask cluster operators to not shut all replicas at once (e.g. during Node Pool upgrades). A Pod Disruption Budget should be provided by the helm chart, for easy and consistent selector settings.

This change is backwards compatible with possible pre-existing PDBs and workflows allowing complete shutdown of clusters, as the flag to emit a PDB definition defaults to false.

Without this change, cluster owners are forced to create a PDB by themselves, manually managing the selector.

Pre-submission checklist:

• [X] Did you explain what problem does this PR solve? Or what new features have been added?
• [X] Have you updated the readme?
• [X] Is this PR backward compatible? If it is not backward compatible, please discuss open a ticket first