core: fix for scheduling cluster custom properties

problem: when adding scheduling custom property to a cluster
and the cluster policy doesn't have this custom policy set, it
will not be save.

fix: should compare the cluster's custom properties with avialable
custom properties and not with cluster policy custom properties.

Change-Id: I8d1a79b3406b577cd94e9f2e46defe7e3554d5a9
Signed-off-by: Gilad Chaplik <gchaplik@redhat.com>