Support message attribute and regexp matching for log resources

Chef 10.18.0 added "message" attribute to the log resource: [CHEF-3006](http://tickets.opscode.com/browse/CHEF-3006). Prefer it for matching, but fallback to "name" attribute on older Chef versions.

Also in addition to a String that has to match the whole log message, allow matching with a Regexp (or any other object that returns true for `object === <log message>`).