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>
Merge pull request #1243 from filbranden/gitignore_thirdparty_etcd1

Add third_party/etcd to .gitignore