engine: fix api/schedulingpolicyunits/policy_unit_id

Change-Id: Ic3cf7127d437506c842b0be93dc4ed29198b7937
Bug-Url: https://bugzilla.redhat.com/1142916
Signed-off-by: Gilad Chaplik <gchaplik@redhat.com>
(cherry picked from commit 0626ac18c35ffaf19708bdd9d313b92282799ed8)