cli: use WARN as the default log level when not explicitly set by user

cli: use WARN as the default log level when not explicitly set by user

For all known log levels (library, CLI, plugins, etc.), when the
dedicated log level environment variable is unset, make the CLI set it
to WARN. This means that you can always override the default, -v, or -d
log level with a specific environment variable. For example, if you need
VERBOSE logging for all the loggers but the library's:

    BABELTRACE_LOGGING_GLOBAL_LEVEL=N babeltrace -d ...

Using WARN as the default log level will help spot bugs quickly without
a specific option to use this log level.

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