Add 'xpath' like handling for variables

[michael-schnell]

Parse configuration with a kind of xpath expressions and provide variables like 'path:attr'.

Example:

"path:" refers to the objects defined in path - In above case a list of "project" objects would be selected. For every object in the list a new variable will be defined with a name like "x_project1", "x_project2" where "name" is an attribute in the selected project instance.