- Move site check for prefix4 and extra_prefixes6 to gluon-core, so the
  rules don't need to be duplicated in several packages. This also fixes
  gluon-respondd not checking extra_prefixes6 at all when
  gluon-ebtables-source-filter is not installed as well.
- A redundant check for prefix6 is removed from gluon-l3roamd (this was
  already checked by gluon-core)
- A separate check for prefix4 remains in gluon-client-bridge, as the
  setting in mandatory there

* add check_site.lua to validate required field from site
* make use of node_client_prefix6 if defined in site