Add a 'flag' argument to the 'dune_python_install_package' command which contains cmake variables to store in metadata file.
This is to be used to fix for example the compiler to use for dune-py but that hasn't been added to the builder yet.
Also changed builddirs output to contain ';' separator (using a cmake string instead of a list).

added 'cmake flags' from metadata file to CMakeLists.txt.template

correctly throw exception if dune-py not available in assertHave