Bump min Docker version

And python-docker version.
Make them less scary.

Change-Id: I134e8aa8ed5eae479e5af743464dc2a0e40a1353

ansible/roles/prechecks/vars/main.yml

 ---
-docker_version_min: '1.10.0'
-docker_py_version_min: '2.0.0'
+docker_version_min: '18.09'
+docker_py_version_min: '3.4.1'
 ansible_version_min: '2.9'
 ansible_version_max: '2.10'
 

releasenotes/notes/bump-minimum-docker-version-18-09-37af3b4c134da67e.yaml

+---
+upgrade:
+  - |
+    Bumps minimum required Docker version to 18.09 and minimum required
+    Docker Python SDK version to 3.4.1. These two are checked in
+    prechecks.