DAOS-16350 test: decrease pool size for ior_per_rank (#15183)

Test deployment/ior_per_rank fails with 'No space' on some CI
clusters. Reduce the requested pool size to accommodate nodes
with smaller storage capacity.

Signed-off-by: James A. Nunez <james.nunez@intel.com>

src/tests/ftest/deployment/ior_per_rank.yaml

@@ -22,7 +22,7 @@ server_config:
       storage: auto
 pool:
   mode: 146
-  size: 750G # Cannot use percentage, as it does not work when using pool create for per rank.
+  size: 350G # Cannot use percentage, as it does not work when using pool create for per rank.
   properties: ec_cell_sz:128KiB
 container:
   type: POSIX