validate define concat params + split warn param

In addition, the warn param has been split into a warn (a bool) and
warn_message (a string).
validate define concat::fragment params
add tests for class concat::setup
add stdlib to rspec-system tests
remove puppet 0.24 support

Introduction of stdlib functions makes support of puppet < 2.7
impractical.
make class concat::setup a private class
use $owner instead of $id to avoid confusion with $::id
mv spec/defines -> spec/unit/defines

To be more consistent with other PL modules