engine: cluster policy commands and queries

Commands:
- Insert
- Update
- Delete
Queries:
- get all cluster policies
- get all policy units
- get all attached clusters (to cluster policy)

Change-Id: I951f1e0c53919cf88f3983d78b96f82b8f20ebcb
Signed-off-by: Gilad Chaplik <gchaplik@redhat.com>