Rename docker directory to heat_docker

This directory was renamed in 4e8231, which made importing docker
(that is docker-py) difficult if not impossible in Python 2.x. So
rename it to something other than 'docker' to avoid that problem.

(setup.cfg has been modified with the new path)

Change-Id: I3cbb7c5d4b53f70e1dc6102881f8e05852341a9a
Related-bug: #1296758
Merge "Rename docker directory to heat_docker"