Add rbac for network managers

[lpichler]

• use AR model to get the result
• add RBAC

introduced in #1343

@miq-bot add_label rbac, technical-dept

@miq-bot assign @martinpovolny
cc @himdel

for example it is in:

Next (if you are not against)

• transform the file to the controller mixin
• refactor with 'DRY' move this code to a method in the mixin
  for example from : https://github.com/ManageIQ/manageiq-ui-classic/blob/master/app/controllers/network_router_controller.rb#L97

  network_managers.each do |network_manager|
      @network_provider_choices[network_manager.name] = network_manager.id
    end

[miq-bot]

@lpichler Cannot apply the following label because they are not recognized: technical-dept

[miq-bot]

Checked commits lpichler/manageiq-ui-classic@6225a3d~...4986c5c with ruby 2.2.6, rubocop 0.47.1, and haml-lint 0.20.0
1 file checked, 0 offenses detected
Everything looks fine. 👍

[dclarizio]

@lpichler can you recommend the fine and euwe labels? I'm not sure if they should follow the original bug fix PR that you reference in the first comment. If so, should probably reference that same BZ or maybe create a new one for this PR. Thx, Dan