Return empty AR relation instead of nil

InfraProvider don't have relation to cloud_tenants - there was
created only empty method returning nil to preserve consitency - but on
some places is expected AR relation object.

For example screen add new subnet, there are listed network managers and
their parent manager's cloud_tenants but they can be infra manager whithout
cloud_tenants.

Such place, where was expected AR Relation object is here:
https://github.com/ManageIQ/manageiq-api/blob/master/app/controllers/api/subcollections/cloud_tenants.rb#L5

which was reached for example by this request:

http://localhost:3000/api/providers/:id/cloud_tenants but it fails later
when object has been infra manager and the method cloud_tenants_query_resource
was returning nil.

app/models/manageiq/providers/openstack/infra_manager.rb

@@ -31,6 +31,7 @@ def availability_zones
   end
 
   def cloud_tenants
+    self.class.none
   end
 
   def host_aggregates

spec/models/manageiq/providers/openstack/infra_manager_spec.rb

@@ -149,4 +149,10 @@
       expect(ems.supported_catalog_types).to eq(%w(openstack))
     end
   end
+
+  let(:openstack_infra_manager) { FactoryGirl.create(:ems_openstack_infra_with_authentication) }
+
+  it 'returns empty relation instead of nil when cloud_tenants are requested on infra provider' do
+    expect(openstack_infra_manager.cloud_tenants).to eq(ManageIQ::Providers::Openstack::InfraManager.none)
+  end
 end