standalone_test: unset log level on root logger

Other modules may have messed with the root logger already, so if we
don't reset the root logger log level, messages go through the logger
even after we add the console and file handlers later.

This problem wasn't visible before because an autotest module was
unconditionally setting the root logger log level to DEBUG, but a recent
patch I sent to autotest removes that unconditional logging setup code.

This patch also renames configure_logging() to reset_logging(), as it
doesn't configure any handler, but just remove the existing logging
configuration).

Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>