haproxy: T5222: Enable backend completion in service ruleset

Enable completion for backend in haproxy service ruleset like so:

```
set load-balancing haproxy service NAME rule 10 set backend
```

Author: indrajitr

interface-definitions/include/haproxy/rule-frontend.xml.i

@@ -50,6 +50,9 @@
               <regex>[-_a-zA-Z0-9]+</regex>
             </constraint>
             <constraintErrorMessage>Server name must be alphanumeric and can contain hyphen and underscores</constraintErrorMessage>
+            <completionHelp>
+              <path>load-balancing haproxy backend</path>
+            </completionHelp>
           </properties>
         </leafNode>
       </children>