kubeadm: handle multiple members without names during concurrent join

For the etcd client, amend AddMember() to handle a very
rare bug when multiple members can end up with the same
name. Match the member peer address and assign it the name of
the member we are adding. For the rest of the members with missing
names use their member IDs as name. The etcd node is not disrupted
by the unknown names.

The important aspects are:
- The number of members of the initial cluster must match
the members in the cluster.
- The member we are current adding is present in the initial cluster.

cmd/kubeadm/app/util/etcd/etcd.go

@@ -303,12 +303,20 @@ func (c *Client) AddMember(name string, peerAddrs string) ([]Member, error) {
 	// Returns the updated list of etcd members
 	ret := []Member{}
 	for _, m := range resp.Members {
-		// fixes the entry for the joining member (that doesn't have a name set in the initialCluster returned by etcd)
-		if m.Name == "" {
-			ret = append(ret, Member{Name: name, PeerURL: m.PeerURLs[0]})
-		} else {
-			ret = append(ret, Member{Name: m.Name, PeerURL: m.PeerURLs[0]})
+		// If the peer address matches, this is the member we are adding.
+		// Use the name we passed to the function.
+		if peerAddrs == m.PeerURLs[0] {
+			ret = append(ret, Member{Name: name, PeerURL: peerAddrs})
+			continue
 		}
+		// Otherwise, we are processing other existing etcd members returned by AddMembers.
+		memberName := m.Name
+		// In some cases during concurrent join, some members can end up without a name.
+		// Use the member ID as name for those.
+		if len(memberName) == 0 {
+			memberName = strconv.FormatUint(m.ID, 16)
+		}
+		ret = append(ret, Member{Name: memberName, PeerURL: m.PeerURLs[0]})
 	}
 
 	// Add the new member client address to the list of endpoints