Hide "Add New Network Manager" unless Nuage is enabled

Nuage is somewhat special in a way it allows adding NetworkManager
directly, without CloudManager. For all other providers such behavior
is not feasible, therefore "Add New Network Manager" button should be
hidden.

With this commit we make sure that the button is hidden unless
config/permissions.yaml file contains entry allowing user to use
Nuage provider:

```
- ems-type:nuage_network
```

BZ: https://bugzilla.redhat.com/show_bug.cgi?id=1577888

Signed-off-by: Miha Pleško <[email protected]>

app/helpers/application_helper/button/ems_network.rb

@@ -1,9 +1,10 @@
 class ApplicationHelper::Button::EmsNetwork < ApplicationHelper::Button::Basic
   def visible?
     # Nuage is only provider that supports adding NetworkProvider manually
-    # therefore we hide drop-down option completely unless Nuage is enabled.
-    return ::Settings.product.nuage unless @record # add new
+    # therefore we hide drop-down option completely unless Nuage is listed
+    # in config/permissions.yaml.
+    return Vmdb::PermissionStores.instance.supported_ems_type?('nuage_network') unless @record # allow add new network manager
 
-    @record.supports?(:ems_network_new) # edit
+    @record.supports?(:ems_network_new) # allow edit of existing manager
   end
 end