tests: increase journal_size value

tests: increase journal_size value

Looks like we are still seeing issue [1].
Let's increase this value to unlock the CI (however, it still needs to
be investigated).

Typical error (see [1] for further details) :
```
[root@osd2 ~]# ceph-volume --cluster ceph lvm batch --filestore --yes --journal-size '2048' /dev/sda /dev/sdb --journal-devices /dev/sdc
Running command: /sbin/vgcreate --force --yes ceph-journals-817ef90b-77ac-4f52-b8a9-30893849fb78 /dev/sdc
 stdout: Physical volume "/dev/sdc" successfully created.
 stdout: Volume group "ceph-journals-817ef90b-77ac-4f52-b8a9-30893849fb78" successfully created
--> Refusing to continue with configured size for journal
-->  RuntimeError: journal sizes must be larger than 2GB, detected: 1024.00 MB
```

[1] https://tracker.ceph.com/issues/41374

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