MXS-2121: Allow destruction of static listeners

If a listener is defined in a static configuration file, it can now be
destroyed at runtime. If MaxScale is restarted, the listener will be
created again unless the configuration file is modified.

server/core/test/rest-api/test/service.js

@@ -165,7 +165,7 @@ describe("Service", function() {
 
     it("destroy a static listener", function() {
         return request.delete(base_url + "/services/RW-Split-Router/listeners/RW-Split-Listener")
-            .should.be.rejected
+            .should.be.fulfilled
     });
 
     after(stopMaxScale)