Fix: make Python plugin provider have its own log level

Fix: make Python plugin provider have its own log level

It cannot share libbabeltrace's log level because it's in a different
shared object itself. Because it's not using the library's global log
level, we don't initialize it with the BABELTRACE_LOGGING_GLOBAL_LEVEL,
but with its own BABELTRACE_PYTHON_PROVIDER_LOG_LEVEL.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>