engine: policy unit should contain single logic

Utill now policy unit could hold logic for filtering, weighing or
balancing (one or more). in this patch each policy unit could hold a
single logic, according to a type field (enum) added to the entity.

Change-Id: Ia6f1d88c7c9bbc600dba0199a7cab27d914c68f4
Signed-off-by: Gilad Chaplik <gchaplik@redhat.com>