Properly handle bootstrap script in seed and lxc modules.

This is related to be605bc81a43879c15f5b026f5d2529a9b008c7e and #14865.
use randomized & restricted dirs for bootstrap
pep8
doc for config.prep_bootstrap
Merge pull request #15545 from makinacorpus/b1

Properly handle bootstrap script in seed and lxc modules.