Add a plugin for monitoring zookeeper

This adds a simple plugin to read data from Zookeeper's MNTR
command to allow for easy monitoring of Zookeeper from collectd
without requiring jmx
Add information to collectd.conf (5) for the zookeeper plugin
Use generic types for a few more of the zookeeper metrics
Merge pull request #826 from katzj/zookeeper

Add a plugin for monitoring zookeeper