Log: change sbd's default logging level to LOG_NOTICE

With the refactoring of logging parts and 1ee3503c, sbd became too
silent given the default logging level LOG_WARNING, even under the
situations where it's supposed to tell something.

This commit changes sbd's default logging level to LOG_NOTICE.
Meanwhile pacemaker library's logging level remains at LOG_WARNING.
With "-v", sbd's logging level is set to LOG_INFO.
With "-vv", sbd's logging level is set to LOG_DEBUG.
With "-vvv", both sbd's and pacemaker library's logging levels are set
to LOG_DEBUG.