Make naming of test-nodes consistent and positive: m0, m1, m2

The nodes used to be named: m-1, m0, m1, that was generating very confusing logs
in integration tests.

tests/framework/integration/cluster.go

@@ -259,7 +259,7 @@ func (c *Cluster) mustNewMember(t testutil.TB) *Member {
 	c.LastMemberNum++
 	m := MustNewMember(t,
 		MemberConfig{
-			Name:                        fmt.Sprintf("m%v", memberNumber-1),
+			Name:                        fmt.Sprintf("m%v", memberNumber),
 			MemberNumber:                memberNumber,
 			AuthToken:                   c.Cfg.AuthToken,
 			PeerTLS:                     c.Cfg.PeerTLS,