ipatests: use whole date when calling journalctl --since
The test test_commands.py::TestIPACommand::test_ssh_key_connection
is checking the content of the journal using journalctl --since ...
but provides only the time, not the whole date with year-month-day.
As a consequence, if the test is executed around midnight it may
find nothing in the journal because it's looking for logs after 11:50PM,
which is a date in the future.
The fix provides a complete date with year-month-day hours:min:sec.
Fixes: https://pagure.io/freeipa/issue/8728
Reviewed-By: Francois Cami <fcami@redhat.com>