MXS-1929: Load global configuration as soon as possible

There exists a dependency on the configuration for the workers: the total
number of worker thread is defined by the `threads` parameter. This means
that the global configuration section must be read before workers are
started.

Commit 411b70e25656317909e54f748f8012593120041f broke MaxScale and turned
it into a single threaded application as the default configuration value
of one worker was used.

server/core/gateway.cc

@@ -1890,6 +1890,12 @@ int main(int argc, char **argv)
         }
     }
 
+    if (!config_load_global(cnf_file_path))
+    {
+        rc = MAXSCALE_BADCONFIG;
+        goto return_main;
+    }
+
     if (daemon_mode)
     {
         if (!change_cwd())