Fix issue15 private registry support

[antoinetran]

Summary

All described in the related issue description.
Tested working in HPC where private registry is mandatory, and direct docker pull is not possible.

Related issue :
#15

[dciangot]

@antoinetran LGTM. I need to check where the impact is in the documentation and installer script. I cannot release it without having the changes ready also there.

[antoinetran]

Also includes a fix in values.yaml and regression fix of kubernetesApiCaCrt indent

[antoinetran]

Includes an improvement over the slurm config yaml in the values.yaml: it can either be a string (as before to not break compatibility), but also a dict. The purpose is to be able to override this config particular values.yaml with others. Without this, there would be duplication of the whole slurm config yaml. Example with the improvement:
values.yaml

plugin:
  config: # <- not a string anymore
    SingularityPath: "singularity"
    ExportPodData: true
    DataRootFolder: ".local/interlink/jobs/"
...

values-somehpc1.yaml

plugin:
  config: # <- not a string anymore
    # Any comment
    DataRootFolder: "/new/path/specific/to/hpc1"

would result in a merge of both plugin config. The drawback is that the comments or ordering are lost in the rendered file but this is not really important usually.