pass node_roles as parameter for plugin_neutronnsx class

- $nodes_hash is not fully qualified variable and it gets undefined, due
  failed puppet dynamic variable lookup, instead of using this bad
  practice we can refer to fully qualified variable
  $::fuel_settings['nodes']; but this couples class with global variables
  which is also not good;
- decouple global hash $::fuel_settings from 'plugin_neutronnsx' class,
  module; pass array of node roles as parameter for class

Change-Id: I868c21fdbe67e59f23f8eca833cd06ab3b22cd96
Closes-bug: #1391956
Merge "pass node_roles as parameter for plugin_neutronnsx class"