Bug #24168 mysql-test-run not functioning in autotest (cannot use external cluster)

- Removed the global variable(s) glob_use_running_cluster/slave
   and put a property on each "cluster" called "use_running" which
   is set to 1 if user passed either ndb_connectstring or 
   ndb_connectstring_slave. Thus mtr.pl should not start/stop the
   cluster.

mysql-test/lib/mtr_process.pl

@@ -385,11 +385,13 @@ sub mtr_kill_leftovers () {
 
   if ( ! $::opt_skip_ndbcluster )
   {
-    # Start shutdown of clusters.
-    mtr_debug("Shutting down cluster...");
 
     foreach my $cluster (@{$::clusters})
     {
+
+      # Don't shut down a "running" cluster
+      next if $cluster->{'use_running'};
+
       mtr_debug("  - cluster " .
 		"(pid: $cluster->{pid}; " .
 		"pid file: '$cluster->{path_pid})");