core: disable when external scheduler is down

- When we are unable perform discovery on the external
scheduler, mark all external policy units as disabled.

Change-Id: I2cd7aa42f85f261f9e55e057b52d7dc4c5738252
Bug-Url: https://bugzilla.redhat.com/1014700
Signed-off-by: Noam Slomianko <nslomian@redhat.com>
Signed-off-by: Gilad Chaplik <gchaplik@redhat.com>