etcdctl: allow move-leader to connect to multiple endpoints with TLS

Re-opening closed PR #11775 which was originaly authored by benmoss.

The mustClientForCmd function is responsible for parsing environment
variables and flags into configuration data. A change was made in #9382
to call Fatal if a flag is provided multiple times. This means that we
cannot call the mustClientForCmd function more than once,
since it will think that flags parsed the first time are now
being redefined and error out.

Some people have commented about this in #8380 but I don't think
there's an open issue for it.

etcdctl/ctlv3/command/move_leader_command.go

@@ -42,7 +42,8 @@ func transferLeadershipCommandFunc(cmd *cobra.Command, args []string) {
 		ExitWithError(ExitBadArgs, err)
 	}
 
-	c := mustClientFromCmd(cmd)
+	cfg := clientConfigFromCmd(cmd)
+	c := cfg.mustClient()
 	eps := c.Endpoints()
 	c.Close()
 
@@ -52,7 +53,6 @@ func transferLeadershipCommandFunc(cmd *cobra.Command, args []string) {
 	var leaderCli *clientv3.Client
 	var leaderID uint64
 	for _, ep := range eps {
-		cfg := clientConfigFromCmd(cmd)
 		cfg.endpoints = []string{ep}
 		cli := cfg.mustClient()
 		resp, serr := cli.Status(ctx, ep)