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