Moving the scripts to a common directory not only vastly simplifies the
zzz-gluon-upgrade script, but also allows to define an ordering of such
scripts across packages.
add invariant script "001-node-system" to add section "system" if not exist; add invariant script "010-node-role"
to add section "system" if not exist; add invariant script "010-node-role" to set role to default value
(site.roles.default) if not exist; depends on new site.conf section
