ipatests: use whole date when calling journalctl --since
    
    The test TestSelfExternalSelf::test_switch_back_to_self_signed
    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.
    Fixes: https://pagure.io/freeipa/issue/8918
    
    Signed-off-by: Florence Blanc-Renaud <flo@redhat.com>
    Reviewed-By: Anuja More <amore@redhat.com>