Issue #551 by pjohans: make sure function exists before executing.
Issue #551 by pjohans:  update feature - make field_openruth_interest_period required to avoid empty option.
Issue #551 by pjohans: Get default_interest period for reservations.
Issue #551 by pjohans: More correct use method openruth_reservation_default_interest_period to get interest period.
Issue #551 by pjohans: Undo make interest_period field required.