Replace bitnami kubectl image with function to create secret

We are currently using the bitnami/kubectl image to communicate with the Kubernetes API to create secrets. However, bitnami is removing their images from DockerHub.

It seems that the failures we are currently seeing in CI are the result of this. While we could replace it with our own image, it is most likely simpler to create a bash function that communicates using the Kubernetes API. This prevents us having to build a separate image for it.