Improve recovering Docker containers when slave is contained

Currently when docker containerizer is recovering it checks the checkpointed executor pids to recover which containers are still running, and remove the rest of the containers from docker ps that isn't recognized.

This is problematic when the slave itself was in a docker container, as when the slave container dies all the forked processes are removed as well, so the checkpointed executor pids are no longer valid.

We have to assume the docker containers might be still running even though the checkpointed executor pids are not.