STORM-498: make ZK connection timeout configurable in Kafka spout

This change is backwards compatible to the previous semantics because Storm's `defaults.yaml` sets the default ZK connection timeout to 15 seconds, which is equal to the previously hardcoded timeout setting in the Kafka spout.

This pull request supersedes the previous pull request https://github.com/apache/storm/pull/256, and fixes the NPE in the `DynamicBrokersReaderTest` test suite.