Add Molecule to Docker role (#5129)

* Add Molecule for container-engine/docker

* Add bootstrap-os to Molecule prepare stage

roles/container-engine/docker/molecule/default/molecule.yml

+---
+driver:
+  name: vagrant
+  provider:
+    name: libvirt
+lint:
+  name: yamllint
+  options:
+    config-file: ../../../.yamllint
+platforms:
+  - name: kubespray-docker
+    box: generic/ubuntu1804
+    cpus: 2
+    memory: 1024
+provisioner:
+  name: ansible
+  env:
+    ANSIBLE_ROLES_PATH: ../../../../
+  config_options:
+    defaults:
+      callback_whitelist: profile_tasks
+  lint:
+    name: ansible-lint
+    options:
+      c: ../../../.ansible-lint
+verifier:
+  name: testinfra
+  lint:
+    name: flake8

roles/container-engine/docker/molecule/default/playbook.yml

+---
+- name: Converge
+  hosts: all
+  become: true
+  roles:
+    - role: kubespray-defaults
+    - role: docker

roles/container-engine/docker/molecule/default/prepare.yml

+---
+- name: Prepare
+  hosts: all
+  gather_facts: False
+  roles:
+    - role: bootstrap-os

roles/container-engine/docker/molecule/default/tests/test_default.py

+import os
+
+import testinfra.utils.ansible_runner
+
+testinfra_hosts = testinfra.utils.ansible_runner.AnsibleRunner(
+    os.environ['MOLECULE_INVENTORY_FILE']).get_hosts('all')
+
+
+def test_docker_service(host):
+    docker = host.service("docker")
+    assert docker.is_running
+    assert docker.is_enabled
+
+
+def test_docker_run(host):
+    with host.sudo():
+        cmd = host.command("docker run hello-world")
+    assert cmd.rc == 0
+    assert "Hello from Docker!" in cmd.stdout