openmpi: add toggle for rsh plm (#30251)

By default `openmpi` needs `rsh` from `openssh`, which is a somewhat
redundant dependency for clusters using slurm. This PR adds a toggle to
allow users to disable the ssh/rsh plm altogether.

var/spack/repos/builtin/packages/openmpi/package.py

@@ -246,6 +246,7 @@ class Openmpi(AutotoolsPackage, CudaPackage):
     variant('lustre', default=False,
             description="Lustre filesystem library support")
     variant('romio', default=True, description='Enable ROMIO support')
+    variant('rsh', default=True, description='Enable rsh (openssh) process lifecycle management')
     # Adding support to build a debug version of OpenMPI that activates
     # Memchecker, as described here:
     #
@@ -329,7 +330,7 @@ class Openmpi(AutotoolsPackage, CudaPackage):
     depends_on('pmix', when='+pmix')
     depends_on('pmix@3.2:', when='@4.0:4 +pmix')
 
-    depends_on('openssh', type='run')
+    depends_on('openssh', type='run', when='+rsh')
 
     conflicts('+cxx_exceptions', when='%nvhpc',
               msg='nvc does not ignore -fexceptions, but errors')
@@ -665,6 +666,10 @@ def configure_args(self):
             # uct btl doesn't work with some UCX versions so just disable
             config_args.append('--enable-mca-no-build=btl-uct')
 
+        # Remove ssh/rsh pml
+        if spec.satisfies('~rsh'):
+            config_args.append('--enable-mca-no-build=plm-rsh')
+
         # some scientific packages ignore deprecated/remove symbols. Re-enable
         # them for now, for discussion see
         # https://github.com/open-mpi/ompi/issues/6114#issuecomment-446279495