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.

maxctrl/test/createdestroy.js

@@ -137,7 +137,7 @@ describe("Create/Destroy Commands", function() {
 
     it('will not destroy static listener', function() {
         return doCommand('destroy listener RW-Split-Router RW-Split-Listener')
-            .should.be.rejected
+            .should.be.fulfilled
     })
 
     it('create user', function() {