Add support for private registry on Megalos

[GioBar00]

- What I did

• Added new Kubernetes setting private_registry_dockerconfigjson for a base64-encoded docker configuration.
• When creating a namespace, if the setting above is set, create also a Kubernetes secret named private-registry of type kubernetes.io/dockerconfigjson.
• When creating the machines, if the setting above is set, add the image_pull_secret argument to the pod specification referring to the private-registry secret.

- How I did it

• Edited KubernetesSettingsAddon.py to add the new setting private_registry_dockerconfigjson.
• Edited KubernetesOptionHandler.py to add the cli menu to enter or delete the setting.
• Edited KubernetesNamespace.py to create the private-registry secret if the setting is set right after creating the namespace.
• Edited KubernetesMachine.py to add the image_pull_secret argument to the pod specification referring to the private-registry secret if the setting is set.

- How to verify it
Create a private registry and get the docker config. Example of the file structure:

{
    "auths": {
        "https://index.docker.io/v1/": {
            "auth": "c3R...zE2"
        }
    }
}

Compute the base64 encoding of the docker config and add it to Kathara settings.
Deploy a lab with kathara lstart or a machine with kathara vstart with an image available on the private registry.

- Description for the changelog

Added support for private registries on Megalos

Resolves #283.

[GioBar00]

- Why
It allows to use images that someone might not want to give public access to.
It is necessary to add this feature directly to Kathará since there is no straight forward way to do it separately.
Kubernetes secrets are namespaced so one would need to create the Kubernetes secret after the namespace is created, but before the pods are deployed. This is not possible when deploying a lab in Kathará since the machines are created right after the namespace is.

[Skazza94]

Hi @GioBar00.
thanks for the PR and sorry for the delay.

I suggest some changes on your code that I would like to discuss before merging the PR.

Thanks,
Mariano.

[GioBar00]

I modified the code with the suggestions above.

[Skazza94]

Hi @GioBar00,
thank you very much!

I will check the changes tomorrow and if everything is ok I will merge the PR in the develop branch for the next release.