provide UI control for NSX plugin 'replication_mode' setting

- NSX cluster can operate without NSX Service nodes, but this requires
  configuration of `replication_mode' setting, so lets add an
  opportunity for user to specify that his NSX cluster runs in testing
  mode (without Service nodes).

Change-Id: Ia545dd90fd7e8b3b2de7280088bd07c689b1adb7
Partial-bug: #1365043
Merge "provide UI control for NSX plugin 'replication_mode' setting"