ModeShape configuration variables should use environment variables, too

ModeShape currently replace variables with Java's system properties, which are set on the java command line using the -Dpropertyname=value syntax. However, it should also look at the environment variables (e.g., System.getenv() so that variables like "${CATALINA_HOME}" can also be used in the ModeShape configuration files without having to define them.

The only downside of doing this is that IIRC Infinispan would not understand environment variables, and it might be confusing that ModeShape understands them when ISPN doesn't. Thoughts?