The boot order for peers is -discovery, -peers, log data, forming
new cluster itself.
Special rules:
1. If discovery succeeds, it would find peers specified by discover URL
only.
2. Etcd would fail when meeting bad -discovery, no -peers and log data.
Add TestDiscoveryDownNoBackupPeersWithDataDir as the test.
Use PUT /_state?prevExist=true in the protocol instead of PUT
/_state?prevValue=init. This lets people point one vanilla etcd at the
key prefix of another vanilla etcd and have it just work.
This tests a variety of failure cases for the Discovery service
including:
- Initial leader failures
- Discovery service failures
- Positive tests for discovery working flawlessly
