This method encapsulates the logic for whether we should use a
JettyAdminServer (i.e., the AdminServer is enabled) or a DummyAdminServer
(i.e., the AdminServer is disabled). It uses reflection when attempting
to create a JettyAdminServer, rather than referencing the class directly,
so that it's ok to omit Jetty from the classpath if a user doesn't wish
to pull in Jetty with ZooKeeper.