Adding test for single port firewall rule

see issue https://issues.apache.org/jira/browse/JCLOUDS-661
Almost trivial comment, but do we need `difference` here? Does [`assertEquals`](http://testng.org/javadoc/org/testng/Assert.html#assertEquals(java.util.Set, java.util.Set)) not work? E.g.
```
assertEquals(expectedPermissions, newGroup.getIpPermissions());
```
? That should also create the appropriate failure message automatically.