engine: cluster policy db support

Added tables:
- cluster policies
- policy units
- cluster policy units

Altered table:
- VdsGroup (reference for cluster policy, and policy custom parameters)

Change-Id: Ibc34c5eba1d68e0650df38053c97e5138de3dfad
Signed-off-by: Gilad Chaplik <gchaplik@redhat.com>