Update PRRTE git submodule to point to Open MPI fork

Shift away from the upstream PRRTE (https://github.com/openpmix/prrte)
and point to an Open MPI-specific fork for future releases of Open
MPI.  The direct consequence of this is that the PRRTE bundled in Open
MPI will now be an Open MPI-specific fork of PRRTE.

Tighter integration between Open MPI and this bundled, Open
MPI-specific PRRTE will likely happen over time, but right now, it's
functionallty identical as the upstream PRRTE.

Note: this commit both updates to the current upstream PRRTE master
branch tip (5cde35d439) plus one additional Open MPI-specific commit
(1d867e8498) that edits the PRRTE VERSION file to include "ompi" to
the version string.

Signed-off-by: Jeff Squyres <jeff@squyres.com>

Author: jsquyres

.gitmodules

@@ -1,6 +1,6 @@
 [submodule "prrte"]
 	path = 3rd-party/prrte
-	url = ../../openpmix/prrte
+	url = ../../open-mpi/prrte
 	branch = master
 [submodule "openpmix"]
 	path = 3rd-party/openpmix