Update github.com/coreos/etcd to v3.3.15

Change-Id: I1b16ca712238219d082427c75dd6bc404794abbf

vendor/github.com/coreos/etcd/raft/logger.go

@@ -114,7 +114,7 @@ func (l *DefaultLogger) Fatalf(format string, v ...interface{}) {
 }
 
 func (l *DefaultLogger) Panic(v ...interface{}) {
-	l.Logger.Panic(v)
+	l.Logger.Panic(v...)
 }
 
 func (l *DefaultLogger) Panicf(format string, v ...interface{}) {

vendor/github.com/coreos/etcd/raft/raft.go

@@ -663,6 +663,7 @@ func (r *raft) becomePreCandidate() {
 	r.step = stepCandidate
 	r.votes = make(map[uint64]bool)
 	r.tick = r.tickElection
+	r.lead = None
 	r.state = StatePreCandidate
 	r.logger.Infof("%x became pre-candidate at term %d", r.id, r.Term)
 }