Reorganize docker-plugin for consistency

Package and Module names shouldn't have dashes in them, since it makes
it harder to import them (i.e. one cannot use ``import dashed-module``).

Also, moving docker resource under resources for consistency.

Partial-Bug: #1271226
Change-Id: I785b626a99516ebe6444296cf62a78dd96355060

contrib/docker_plugin/tests/test_docker_container.py

@@ -23,7 +23,7 @@ from heat.tests.common import HeatTestCase
 from heat.tests import utils
 
 from .fake_docker_client import FakeDockerClient  # noqa
-from ..plugin import docker_container  # noqa
+from ..resources import docker_container  # noqa
 
 
 template = '''