testimage: Don't use systemd.log_level=debug logging
This causes MBs of log messages which overload the standard 200kb syslog limit
used by busybox syslog by default. This means some tests which rely on messages
being detected in syslog, e.g.
rpm.RpmInstallRemoveTest.test_check_rpm_install_removal_log_file_size
fail (AssertionError: 54 not greater than or equal to 80 : Cound not find sufficient
amount of rpm entries in /var/log/messages, found 54 entries).
We enabled this to aid debugging of some systemd race issues, those are
now resolved so we can disable this. Leave the log level parameter as a
comment to save others having to look it up if they need debugging.
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
