Skip if we don't have rpmbuild
Make python_scripts_executable test more deterministic

It was failing on OSX due to file searching occuring in a different
order, and frankly we can predict the path of the script dir, so
let's use that instead of searching for it.
Remove ftw; not used anymore. (#675)
Attempt to provide better error messages for busted python environments.

This aims to help with #644 where we cannot get python package metadata
without having json and setuptools available from the python program
we are invoking.