engine: cluster policy: adding description and info icons

* A description field is added to each pre-defined policy unit.
* A description will be shown in a tool-tip foreach policy unit in cluster
policy dialog, as well as info icon explaining the various section in the
dialog.

Change-Id: Ib51cf7f15b7d73c87e0c728953171584216e1d00
Signed-off-by: Gilad Chaplik <gchaplik@redhat.com>