added System.getProperties to the VariableResolver

this helps with "mvn package -Denvironment=test" to replace [[environment]] in the control-file.
so the package name could be [[name]]-[[environment]].
this is helpfull for projects with a development and production stage to build and deploy a package for the needed environment without the need of a 2nd branch or changes in the code. and later the same sourcecode can be used to build a package for an other environment.