Implement Placeholder

Registered by Rico Lin on 2016-11-02

POC: https://review.openstack.org/#/c/387463/
This PB is about improving validation process from heat. By introduction a
placeholder, we can prevent any intended `None` as a return value to cause unexpected behaviors.
Also placeholder can record any validation we done with that value. This might help with reducing
duplicated validation.

Blueprint information

Status:
Not started
Approver:
None
Priority:
Low
Drafter:
Rico Lin
Direction:
Needs approval
Assignee:
Rico Lin
Definition:
New
Series goal:
None
Implementation:
Unknown
Milestone target:
None

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