MOS can be used with NSX cluster that lacks Service nodes

- remove warning that says that NSX Service node is mandatory for
  Mirantis OpenStack
- add note about fact that NSX Service node is mandatory for high volume
  environments
- update screenshot with NSX settings block that shows new checkbox

Change-Id: I196f1b6353b8a64ba221d93f0622f6d3859dc9ea
Partial-bug: #1365043
Merge "MOS can be used with NSX cluster that lacks Service nodes"