mtr.pl fixes for ndb

 - Make it possible to use MTR_MAXNDB to set the upper limit of number of
  parallel ndb test to run.
 - Very useful on machines with many cores and lots of RAM

mysql-test/mysql-test-run.pl

@@ -506,8 +506,8 @@ sub run_test_server ($$$) {
   my $num_failed_test= 0; # Number of tests failed so far
 
   # Scheduler variables
-  my $max_ndb= $childs / 2;
-  $max_ndb = 4 if $max_ndb > 4;
+  my $max_ndb= $ENV{MTR_MAX_NDB} || $childs / 2;
+  $max_ndb = $childs if $max_ndb > $childs;
   $max_ndb = 1 if $max_ndb < 1;
   my $num_ndb_tests= 0;