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/unit/plugins/module_utils/_api/utils/test_utils.py

@@ -255,7 +255,7 @@ class ParseEnvFileTest(unittest.TestCase):
 class ParseHostTest(unittest.TestCase):
     def test_parse_host(self):
         invalid_hosts = [
-            '0.0.0.0',
+            'foo://0.0.0.0',
             'tcp://',
             'udp://127.0.0.1',
             'udp://127.0.0.1:2375',