Setting summit env for OOM errors

OLCF recommends setting "OMPI_MCA_io" env variable to "romio314"
to prevent out of memory issues with the code. In our testing we
have found that setting this env gets rid of OOM errors with
certain E3SM simulations (ECP simulation runs).

However also note that setting this environment variable reduces
the performance of parallel HDF5 (when using NetCDF4P PIO iotype
to write data).

See Issue #2856

cime/config/e3sm/machines/config_machines.xml

@@ -3078,6 +3078,8 @@
     </environment_variables>
     <environment_variables mpilib="!mpi-serial">
       <env name="HDF5_PATH">$ENV{OLCF_HDF5_ROOT}</env>
+      <!-- OMPI_MCA_io env is required due to OOM errors -->
+      <env name="OMPI_MCA_io">romio314</env>
       <env name="PNETCDF_PATH">$ENV{OLCF_PARALLEL_NETCDF_ROOT}</env>
     </environment_variables>
   </machine>