Fix: bt_config_convert_from_args(): set default log level at the end too

Issue
=====
There are code paths which make bt_config_convert_from_args() return a
configuration while not having set the default log level.

For example, with `-o ctf-metadata`, the function calls
bt_config_print_ctf_metadata_create() to create such a configuration,
and then goes to the `end` label).

Solution
========
Make sure it is set to the CLI's default log level value when reaching
the end of the function.

Known drawbacks
===============
None.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: I2ee21a754aeaea65fd6d4fbfcae5bd2f07fcba7c
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1525
Tested-by: jenkins <jenkins@lttng.org>