Drop docker_utils

This was last used by fedora swarm driver, which was dropped in Change
I81095221fa45f7070bde22e2a4415345080ed1f1

Change-Id: Id5e61029d54424e8f3462e5063b1beb2495ec305

Author: waipeng

magnum/common/docker_utils.py

@@ -16,7 +16,6 @@ WebOb>=1.8.1 # MIT
 alembic>=0.9.6 # MIT
 cliff!=2.9.0,>=2.8.0 # Apache-2.0
 decorator>=3.4.0 # BSD
-docker>=4.3.0 # Apache-2.0
 eventlet>=0.28.0 # MIT
 iso8601>=0.1.11 # MIT
 jsonpatch!=1.20,>=1.16 # BSD