engine: add enabled column to policy_units

As part of the External scheduler integration:
- Added "enabled" field to policy unit object, dao, and db

This patch is meant to decrease the size and simplify
the External Scheduler integration patch

Change-Id: I6be407297c259aedf17283d2f674021cf0e4a135
Signed-off-by: Noam Slomianko <nslomian@redhat.com>