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/sanity/extra/action-group.py

@@ -20,6 +20,7 @@ ACTION_GROUPS = {
         'pattern': re.compile('^.*$'),
         'exclusions': [
             'current_container_facts',
+            'docker_context_info',
         ],
         'doc_fragment': 'community.docker.attributes.actiongroup_docker',
     },