adding support for optional config of r10k module_path

- adding support for optional config of r10k module_path
- check if it is defined in puppet provisioner module_path
- be aware that module_path in provisioner could be an array
- if no config option is provided construct a valid path from the puppet provisioner module_path
- if this is an array, use the first element
- output some infos for the user
- of course check stuff in config.rb
- no tests yet... :(