tests/journal_collocation: adds testing values to ceph_conf_overrides

This gives test coverage to changes introduced in:

https://github.com/ceph/ceph-ansible/pull/1214

Signed-off-by: Andrew Schoen <aschoen@redhat.com>

tests/functional/centos/7/journal-collocation/group_vars/all

@@ -12,3 +12,7 @@ journal_collocation: True
 os_tuning_params:
   - { name: kernel.pid_max, value: 4194303 }
   - { name: fs.file-max, value: 26234859 }
+ceph_conf_overrides:
+  global:
+    osd_pool_default_pg_num: 8
+    osd_pool_default_size: 1