Feature/zookeeper user group

This should resolve issues with the `zookeeper` recipe.

Tested on:

Distribution/OS | HDP 2.0.11.0 | HDP 2.1.3.0 | CDH 4.7.0 | CDH 5.1.2
--- | ----- | ----- | ----- | -----
Ubuntu 12.04 | :white_check_mark: | :white_check_mark: | :white_check_mark: | :white_check_mark:
CentOS 6.5 | :white_check_mark: | :white_check_mark: | :white_check_mark: | :white_check_mark: