move registry info as external variables

group_vars/dev_environments.yml.example

-docker:
-  registry_url: gitlab.geant.net:5005/docker/ci_registry
-  username: first.lastname
-  private_token: my-secret-token
-
-nat_user: nat

playbook.yml

 - name: Install Docker and ContainerLab
-  hosts: dev_environments
+  hosts: all
   become: true
+  vars: 
+    docker:
+      registry_url: gitlab.geant.net:5005/docker/ci_registry
+      username: "{{username }}"
+      private_token: "{{ secret_token}}"
+    nat_user: nat
   roles:
     - docker
     - containerlab