Enhancements to availability zones api - JCLOUDS-582

https://issues.apache.org/jira/browse/JCLOUDS-582
Use `new CreateServerOptions()` instead of `null` here, or have a (nasty?) if...then below that does something like
```
if (options != null) {
  ServerCreated server = serverApi.create(hostName, imageIdForZone(zoneId), flavorRefForZone(zoneId), options);
} else {
  ServerCreated server = serverApi.create(hostName, imageIdForZone(zoneId), flavorRefForZone(zoneId));
}
```
?

It's [not marked](https://github.com/jclouds/jclouds/blob/master/apis/openstack-nova/src/main/java/org/jclouds/openstack/nova/v2_0/features/ServerApi.java#L84) as `@Nullable`, so I think we'd be safer not passing `null` here.

@jdaggett @zack-shoylev @everett-toews Please correct me if that's not right!