Support properties instance other than System.getProperties() when resolving configuration placeholders

The current implementation always uses System.getProperties() when resolving configuration placeholders (See InfinispanConfiguration line 331) and this is a bit limiting when trying to configure Infinispan in a webapplication. In this specific case, I need to construct a Properties object with a combination of System.getProperties() and the servlet context parameters. In other cases maybe the web app don't have access permissions to System.getProperties().