nuageinit: Replace os.execute with Lua libraries

Prefer posix.sys.stat's chmod() to os.execute(). While here, change the
name of the locals to be more descriptive.

Signed-off-by: Jose Luis Duran <jlduran@gmail.com>
(cherry picked from commit 9b2d92addc31ba6f5696c85d184a45d43e9073dc)