Revert "engine: add comment field to Cluster [2]"

This reverts commit 1277d0f702fb710f9946ba8fa0d7d4dbdee55aab

found 2 problems.. 
[http://gerrit.ovirt.org/#/c/16589/]

Change-Id: I110cff674a2bf7f1184d0f6d527e7103a04e927a