DPM: Force 'hash' GDS component when singleton spawn

 * See openpmix/openpmix#2705

Signed-off-by: Joshua Hursey <jhursey@us.ibm.com>
(cherry picked from commit 245424c)

ompi/dpm/dpm.c

@@ -23,6 +23,7 @@
  * Copyright (c) 2021      Nanook Consulting.  All rights reserved.
  * Copyright (c) 2018-2022 Triad National Security, LLC. All rights
  *                         reserved.
+ * Copyright (c) 2022      IBM Corporation.  All rights reserved.
  * $COPYRIGHT$
  *
  * Additional copyrights may follow
@@ -1610,6 +1611,11 @@ int ompi_dpm_spawn(int count, const char *array_of_commands[],
     OPAL_LIST_DESTRUCT(&job_info);
 
     if (ompi_singleton) {
+        /* The GDS 'hash' component is known to work for singleton, so
+         * recommend it. The user may set this envar to override the setting.
+         */
+        setenv("PMIX_MCA_gds", "hash", 0);
+        /* Start the DVM */
         rc = start_dvm(hostfiles, dash_host);
         if (OPAL_SUCCESS != rc) {
             if (NULL != pinfo) {