[Matthias schiffer: rebase, add a few more restrictions]

"gluon-node-info": add section system and option role; add announce script to announce the role as nodeinfo;
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