Avoid disruptive changes to Shoot ETCD

[elenz97]

How to categorize this issue?

/area robustness
/area control-plane

What would you like to be added:

In our Gardener setup, we are required to use our own StorageClass for the Shoot ETCD component.
As far as I can understand how a shoot ETCD is provisioned, the StorageClass will always be obtained from the provided config.etcd.storage configuration from the gardener-extension-provider-openstack helm chart. After the initial reconciliation of a shoot cluster, changing the .storage configuration to reference a custom StorageClass could then disrupt existing ETCD deployments, as its volumeClaimTemplates will ultimately be overridden¹.
I'd like to propose adding a safeguard in EnsureETCD()¹ to ensure the configured StorageClass will not be altered when modifying the storage configuration.

Why is this needed:

To not disrupt existing ETCD deployments by modifying the storage configuration via the helm chart.

Footnotes

1. https://github.com/gardener/gardener-extension-provider-openstack/blob/master/pkg/webhook/controlplaneexposure/ensurer.go#L88 ↩ ↩²