restapi: add scheduling policy elements to rsdl_metadata.yaml

Complementing http://gerrit.ovirt.org/#/c/28093/ .
Marking cluster's fields: scheduling_policy.id and
scheduling_policy.thresholds (low, high and duration) as deprecated.

Change-Id: I36f5efe9f82c914e0dec80572b2500e6839e5f7b
Signed-off-by: Gilad Chaplik <gchaplik@redhat.com>