XD-1393 support ZK client connection string
XD-1393 Fix zkClientConnect option

- Fix zk.client.connect option to use empty value instead of ''
- Remove command line options so that this property can only be
  configured either from application.yml or config/xd-config.yml while
  the later has the highest precedence.
  - Fix the related ZookeeperClientConnectTest
- Fix ModuleOptionsDefaultsOrderingTests' application-test.yml to include
  the `zk.client.connect` property