Add docker_context_info module (#1039)

* Vendor parts of the Docker SDK for Python

This is a combination of the latest git version
(https://github.com/docker/docker-py/tree/db7f8b8bb67e485a7192846906f600a52e0aa623)
with some fixes to make it compatible with Python 2.7
and adjusting some imports.

* Polishing.

* Fix bug that prevents contexts to be found when no Docker config file is present.

Ref: https://github.com/docker/docker-py/issues/3190

* Linting.

* Fix typos.

* Adjust more to behavior of Docker CLI.

* Add first iteration of docker_context_info module.

* Improvements.

* Add basic CI.

* Add caveat on contexts[].config result.

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

@@ -0,0 +1,40 @@
+---
+# 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 #
+####################################################################
+
+# Create random name prefix (for containers, networks, ...)
+- name: Create random container name prefix
+  set_fact:
+    cname_prefix: "{{ 'ansible-docker-test-%0x' % ((2**32) | random) }}"
+    cnames: []
+
+- debug:
+    msg: "Using container name prefix {{ cname_prefix }}"
+
+# Run the tests
+- block:
+    - include_tasks: run-test.yml
+      with_fileglob:
+        - "tests/*.yml"
+      loop_control:
+        loop_var: test_name
+
+  always:
+    - name: "Make sure all containers are removed"
+      docker_container:
+        name: "{{ item }}"
+        state: absent
+        force_kill: true
+      with_items: "{{ cnames }}"
+      diff: false
+
+  when: docker_api_version is version('1.25', '>=')
+
+- fail: msg="Too old docker / docker-py version to run all docker_container tests!"
+  when: not(docker_api_version is version('1.25', '>=')) and (ansible_distribution != 'CentOS' or ansible_distribution_major_version|int > 6)