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

contrib/heat_docker/setup.cfg

@@ -20,7 +20,7 @@ classifier =
 [files]
 # Copy to /usr/lib/heat for plugin loading
 data_files =
-    lib/heat/docker = docker/resources/*
+    lib/heat/docker = heat_docker/resources/*
 
 [global]
 setup-hooks =

doc/source/template_guide/contrib.rst

@@ -42,8 +42,8 @@ DockerInc Resource
 This plugin enables the use of  Docker containers in a Heat template and
 requires the `docker-py <https://pypi.python.org/pypi/docker-py>`_
 package. You can find more information in the `README
-<https://github.com/openstack/heat/blob/master/contrib/docker/docker/
-README.md>`_.
+<https://github.com/openstack/heat/blob/master/contrib/heat_docker/heat_docker
+/README.md>`_.
 
 .. resourcepages:: DockerInc::
 
@@ -57,4 +57,4 @@ find more information in the `README
 <https://github.com/openstack/heat/blob/master/contrib/nova_flavor/nova_flavor
 /README.md>`_.
 
-.. resourcepages:: NovaFlavor::
+.. resourcepages:: NovaFlavor::