state the fact that NSX Service node is mandatory for Mirantis OpenStack

- NSX cluster can successfully operate without Service node, but our
  tests show that creation of virtual interfaces fails, due to incorrect
  configuration of `replication_mode' setting in Neutron NSX plugin
  confguration file (plugin is configured to use Service node)
- we need more work to be done to implement working Mirantis OpenStack
  with NSX cluster that operates without Service node
- note that OpenStack itself can be used with NSX cluster that lacks
  Service node
- while here add missing article

Change-Id: Ibd88834a525104f4c9e71c355c3e7e0ea833adba
Closes-bug: #1365043