Vendor API connection code from Docker SDK for Python (#398)

* Vendor parts of the Docker SDK for Python

This is a combination of the latest git version
(https://github.com/docker/docker-py/tree/a48a5a9647761406d66e8271f19fab7fa0c5f582)
and the version before Python 2.7 support was removed
(https://github.com/docker/docker-py/tree/650aad3a5fb84059b392ad450f760ed08143ae3f),
including some modifications to work with Ansible module_utils's
system (i.e. third-party imports are guarded, and errors are
reported during runtime through a new exception
MissingRequirementException).

* Create module_utils and plugin_utils for working with the vendored code.

The delete call cannot be called delete() since that method already exists from requests.

* Vendor more code from Docker SDK for Python.

* Adjust code from common module_utils.

* Add unit tests from Docker SDK for Python.

* Make test compile with Python 2.6, but skip them on Python 2.6.

* Skip test that requires a network server.

* Add changelog.

* Update changelogs/fragments/398-docker-api.yml

Co-authored-by: Brian Scholer <1260690+briantist@users.noreply.github.com>

* Minimum API version is 1.25.

Co-authored-by: Brian Scholer <1260690+briantist@users.noreply.github.com>

plugins/module_utils/_api/transport/basehttpadapter.py

@@ -0,0 +1,19 @@
+# -*- coding: utf-8 -*-
+# This code is part of the Ansible collection community.docker, but is an independent component.
+# This particular file, and this file only, is based on the Docker SDK for Python (https://github.com/docker/docker-py/)
+#
+# Copyright (c) 2016-2022 Docker, Inc.
+#
+# It is licensed under the Apache 2.0 license (see Apache-2.0.txt in this collection)
+
+from __future__ import (absolute_import, division, print_function)
+__metaclass__ = type
+
+from .._import_helper import HTTPAdapter as _HTTPAdapter
+
+
+class BaseHTTPAdapter(_HTTPAdapter):
+    def close(self):
+        super(BaseHTTPAdapter, self).close()
+        if hasattr(self, 'pools'):
+            self.pools.clear()