Previously cleanup-containers suffered from the following
limitations:
1. The docker rm command would fail if all the containers were not
stopped.
2. The script would throw an error if no containers existed.
3. Artifacts such as veths, bridges, etc.. would remain on the host
after containers were removed. These artifacts should be removed
if the containers are removed.
Change-Id: I5eadb23fec9e34125a32f6c3e39c241d97932910
556388e737