Make AbstractYarnClusterDescriptor immutable

AbstractYarnClusterDescriptor should be made immutable. Currently, its internal configuration is modified from different places which makes it difficult to reason about the code. For example, it should not be possible to modify the zookeeperNamespace using a setter method. A user of this class should be forced to provide all information prior to creating the instance, e.g., by passing a org.apache.flink.configuration.Configuration object.