Move python{,3}-docker installation to another task

This also adds support for installing python3-docker (not python-docker)
in systems that run Python 3.

roles/matrix-base/tasks/server_base/setup_raspbian.yml

@@ -28,7 +28,6 @@
 - name: Ensure APT packages are installed
   apt:
     name:
-      - python-docker
       - "{{ matrix_ntpd_package }}"
       - fuse
     state: latest
@@ -38,5 +37,6 @@
   apt:
     name:
       - "{{ matrix_docker_package_name }}"
+      - "python{{'3' if ansible_python.version.major == 3 else ''}}-docker"
     state: latest
   when: matrix_docker_installation_enabled|bool