[all]: optional image registry

[lobis]

Closes #232

This PR adds the option to specify an image registry which will be used to pull images. This is useful for example to avoid rate limiting from docker.io.

Currently the charts have dependence on other images, in many cases these can be specified via the image: so technically we already support registry usage, we just need to update the image url to include the registry. In some cases however the image is not exposed to the user (such as busybox in the webui). This makes it impossible for the user to configure a registry for this image, which may result in a rate limit block.

This PR adds an optional configuration, imageRegistry which defaults to "" and can point to a given registry e.g. registry.cern.ch/docker.io. If this option is set then all images used by the chart will be prepended this registry, even those that can be set manually by users.

Example usage:

  imageRegistry: registry.cern.ch/docker.io

  image:
    repository: rucio/rucio-server
    tag: release-36.2.0

All images, including rucio/rucio-server will be preprended the registry. This also applies to "hidden" dependencies such as busybox, apache, etc.