zun: load container driver from entry point

Starting from U release, container driver is loaded from entry point.
Deployers should specify the entry point (i.e. ``docker``) in config
option ``container_driver`` under ``[DEFAULT]`` session.

Zun patch: https://review.opendev.org/#/c/703259/

Change-Id: I96e38760e7b13a6e11737372e9e7fd36cca6f749