Support System Property substitution in xml-config values

In our current setting, it is required to reference the standard java keystore within betterFORM. The standard java keystore resides in JAVA_HOME, which cannot be configured as absolute path (as this heavily depends on the machine, e.g. on linux systems this might be /usr/java/latest, on Windows systems this might be C:\Program Files\Java, ....).

Apache Tomcat allows substitutions of System properties with the syntax ${propertyName}, e.g. ${java.home}.

There's already a method in StrSubstitutor that does exactly that. No additional dependencies needed.

Test case is included and "green" on my machine (the test ought to succeed on every testing environment that has the property java.home set).