crimson/qa/objectstore-tool: reduce segments size

used in testing. This translates into more segments, which helps
in preventing test failures due to insufficient free segments for mounting.

Signed-off-by: Ronen Friedman <rfriedma@redhat.com>

qa/suites/crimson-rados/objectstore_tool/tasks/crimson_obejctstore_tool.yaml

@@ -1,5 +1,8 @@
 overrides:
   ceph:
+    conf:
+      osd:
+        seastore_segment_size: 16777216
     log-ignorelist:
     - but it is still running
     - overall HEALTH_