Add third_party/etcd to .gitignore

Otherwise the tree looks "dirty" after hack/install-etcd.sh is executed.

Tested:
- Ran `git status` after running `hack/install-etcd.sh`.

Signed-off-by: Filipe Brandenburger <filbranden@google.com>
Convert existing kubernetes system to use ContainerStatus, instead of
docker.Container directly.