Extend String Variable Insertion to also detect and resolve "Bundles.find_file"-pattern

[maltewi]

There are some config files around using orocos.rb's functionality to search for files in bundles. E.g.

path: <%= "#{Bundles.find_file('config/motion_planner/trajopt.yml')}" %>

as reported in https://git.hb.dfki.de/d-rock/cnd-orogen-execution/issues/2.

This could also be resolved by libConfig by mapping to calling this function:

lib_config/src/Bundle.cpp

Line 323 in 78ab9b5

std::string Bundle::findFileByName(const std::string& relativePath)