test: workaround failure in journal.sh

test: workaround failure in journal.sh

With the changes to ensure that the commit position of a new
client is initialized to the minimum position of other clients,
the 'journal inspect/export' commands return zero records because
the master client has committed all of its entries.

Workaround this by restoring the initial commit position after
writing to the image.

Fixes: http://tracker.ceph.com/issues/16011
Signed-off-by: Mykola Golub <mgolub@mirantis.com>