ImagePruner should allow to ignore invalid references

The background is the following RFE: https://issues.redhat.com/browse/RFE-924 and is leveraged by OpenShift Dedicated.

Story: As a cluster administrator I would like to rely on a more robust image pruning mechanism so that pruning commences even in the face of invalid image references.

Acceptance criteria:

• the Image Registry Operator should support the `--ignore-invalid-refs` flag to be passt on to the image pruner
• the option should be enabled by default for new clusters
• the option should remain disabled for existing clusters