feat(discovery): adjust boot order to find peers

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.
docs: boot order to find peers