Rollback a zone and related records to a prior version

Upon reviewing prior versions of a zone and related records. A user can then choose a specific prior version and notify the system to update the zone to that version's state.

Example:
User A made a change to record Y on zone X.
User X created zone T with records B, C and D.

An option to define the time period/number of changes to keep is also needed.

Design note:
 - The capability would need an API spec, as it should be accessible to users via API
 - Database implications
 - Ceilometer is considered useful for events the operator cares about (e.g. to track billing changes). It is not necessarily considered a viable implementation for this.
 - Use the "state" model vs CRUD for "undoing"