tests: increase `mon_max_pg_per_osd`

tests: increase `mon_max_pg_per_osd`

we aren't deploying enough OSD daemon, so it fails like following:

```
  stderr: 'Error ERANGE: pool id 10 pg_num 256 size 2 would mean 1536 total pgs, which exceeds max 1500 (mon_max_pg_per_osd 250 * num_in_osds 6)'
```

Let's increase the value of `mon_max_pg_per_osd` in order to get around
this issue in the CI.

Signed-off-by: Guillaume Abrioux <gabrioux@redhat.com>