[#3699] Allow customization of repositories

[ljupcovangelski]

The output on helm template renders the ConfigMap like this:

# Source: airy/templates/controller/configmap.yaml
kind: ConfigMap
apiVersion: v1
metadata:
  name: repositories
  namespace: 
data:
  repositories.json: |
    {
      "repositories":
            [
        {
          "name": "airy-core",
          "password": null,
          "url": "https://helm.airy.co",
          "username": null
        }
      ]
    }

Tested also with a list of multiple repositories.

Resolves #3699 .

[ghost]

Looks good!