Handle Secret containing a docker config file

[flavio]

This is related with https://github.com/kubewarden/kubewarden-controller/issues/42

We need to offer the following scenarios via our helm chart:

1. User has already a Kubernetes Secret that contains the docker configuration file that allows pull operations. The user just enters the name of the Secret into values.yml -> The helm chart writes the name of the Secret inside of the policy-server ConfigMap
2. The user provides the contents of the docker configuration file via the values.yml file -> the helm chart creates a Secret and then adds a reference to it inside of the policy-server ConfigMap
3. When none of the previous scenarios happen, no Secret is referenced inside of the ConfigMap. This is the current behavior