Zk kafka plugins

Add a zookeeper config plugin to load json string config from zookeeper;

Add a kafka log receiver plugin, the plugin can only send messages less then 900 Bytes each time. The plugin is not actually practical at the moment, may someone help to figure out a proper way to code with LibKafka.

Yours,
Imcom
You can define the `kafka_port` flag as an int and then you won't have to cast here (and gflags will enforce type safety).