Add docker_image_pull module (#728)

* Add docker_image_pull module.

* Support platform during idempotency check.

* Add diff mode, extend tests.

* Add image ID tests.

tests/integration/targets/docker_image_pull/tasks/main.yml

@@ -0,0 +1,13 @@
+---
+# Copyright (c) Ansible Project
+# GNU General Public License v3.0+ (see LICENSES/GPL-3.0-or-later.txt or https://www.gnu.org/licenses/gpl-3.0.txt)
+# SPDX-License-Identifier: GPL-3.0-or-later
+
+####################################################################
+# WARNING: These are designed specifically for Ansible tests       #
+# and should not be used as examples of how to write Ansible roles #
+####################################################################
+
+- when: ansible_facts.distribution ~ ansible_facts.distribution_major_version not in ['CentOS6', 'RedHat6']
+  include_tasks:
+    file: test.yml