[JENKINS-36544] Fix Node Properties on Jenkins 2.205+

This is a follow-up fix for PR jenkinsci#382.

In 2.204, the "Configure Clouds" screen was on the main /configure system
configuration page. As a result, `${it}` referred to the global system
object, aka the Hudson instance.

In 2.205, PR jenkinsci/jenkins#4339 merged, which moves the Configure Clouds
section to a new page: /configureClouds. As a result of that change, `${it}`
no longer refers to the Hudson instance, but to a new
GlobalCloudConfiguration object. That object does not have the
`getNodePropertyDescriptors()` method, so it is not able to populate the
"Node Properties" block.

Switching that to `${app}` gives it a correct reference to the Hudson object,
and that allows it to show the Node Properties block on all Jenkins versions,
both before and after v2.205.

src/main/resources/hudson/plugins/ec2/SlaveTemplate/config.jelly

@@ -214,7 +214,7 @@ THE SOFTWARE.
       <f:textbox default="-1"/>
     </f:entry>
 
-    <f:descriptorList title="${%Node Properties}" field="nodeProperties" descriptors="${it.getNodePropertyDescriptors()}" />
+    <f:descriptorList title="${%Node Properties}" field="nodeProperties" descriptors="${app.getNodePropertyDescriptors()}" />
 
   </f:advanced>