Add ServletContextPropertyUtils 

Prior to this commit the Log4JWebConfigurer used the SystemPropertyUtils to resolve placeholders. The ServletContextPropertyUtils adds the ability to use the ServletContext to resolve placeholders as well.

The usecase for this is that we use a ServletContextListener to dynamically add some init-parameters to the ServletContext, those are needed to determine the location of the LOG4J configuration file.

Issue: 10284

I've signed the CLA.