Make operator resilient to registry failure

With the upcoming Gitlab OTG coming, considerations towards how we handle image pulling were discussed between me and @jhensche .

This MR comes as a proposal of improvement to avoid application downtime due to container registry unavailability, by using a default pulling that will take advantage of the node cached image, and only resort to pull the image when not available in the node.

Part of cern-drupal-distribution#19 (closed) Part of https://gitlab.cern.ch/drupal/overview/-/issues/10