Comment 0 for bug 1772673

Similiar to https://bugs.launchpad.net/tripleo/+bug/1772071.

The script that writes /etc/hosts relies on environment variables to pass the list of host entries: http://git.openstack.org/cgit/openstack/tripleo-heat-templates/tree/scripts/hosts-config.sh?id=1eaa019a1f1fbf126c8d9cbffdb608a02d96f3f7#n38

On large scale deployments this could exceed the maximum environment block size. Refactoring this to use str_replace in head should avoid the limit.