Fix ControlPlane reconciliation for K8s < 1.19

[ialidzhikov]

/kind bug

After #205 the ControlPlane reconciliation for K8s < 1.19 fails with:

  lastErrors:
    - description: >-
        task "Waiting until shoot control plane has been reconciled" failed:
        Error while waiting for ControlPlane shoot--foo--bar/bar
        to become ready: extension encountered error during reconciliation:
        Error reconciling controlplane: could not apply control plane chart for
        controlplane 'shoot--foo--bar/bar': could not inject
        chart 'csi-driver-controller' images: could not find image
        "csi-driver-cinder" opts runtime version v1.18.14 target version 1.18.14
      taskID: Waiting until shoot control plane has been reconciled

Although the csi-driver is deployed only for K8s > 1.19, the csi-driver-cinder image is part of the controlPlaneChart - ref https://github.com/gardener/gardener-extension-provider-openstack/blob/v1.15.0/pkg/controller/controlplane/valuesprovider.go#L173. Hence when the controlPlaneChart is being applied, it tries to find the corresponding csi-driver-cinder image even for K8s < 1.19 even though the corresponding csi chart won't be applied.
This PR contains a straightforward fix.

Release note:

NONE

[ialidzhikov]

/cc @dkistner @kon-angelo

[rfranzke]

/lgtm

[kon-angelo]

I am am doing a test run right now and I will deploy to dev once merged.

[kon-angelo]

/lgtm