Bad System.out.println logging usage
Description
The xslt extension uses System.out.println() to print debugging info instead of properly using geoserver logging, which can uselessly fill out catalina.out.
While i can understand system.out.println() being used in tests, it shouldn't appear in production code..
https://github.com/geoserver/geoserver/blob/master/src/extension/xslt/src/main/java/org/geoserver/wfs/xslt/XSLTOutputFormat.java#L83 is trigerred quite often here, and https://github.com/geoserver/geoserver/blob/master/src/extension/xslt/src/main/java/org/geoserver/wfs/xslt/XSLTOutputFormat.java#L95 happens too.
https://github.com/geoserver/geoserver/search?utf8=%E2%9C%93&q=%22System.out.println%22&type=Code
Environment
Tomcat 8, debian jessie
