cli: use log level integer instead of letter in configuration

cli: use log level integer instead of letter in configuration

This patch makes `struct bt_config` have an integer log level (to be set
to one of the `BT_LOG_*` values) instead of a letter. The
bt_log_get_letter_from_level() and bt_log_get_level_from_letter() can be
used to translate from/to a log level letter/integer, so we use that.

This is just cleaner and makes more sense than carrying a letter in the
configuration which is just one encoding of a log level value.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: I5cbc3f2cb9ac868bc99e9458468a09c58e716884
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1452
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Francis Deslauriers <francis.deslauriers@efficios.com>