zone in multiple backend pools

A zone can be exported from one pool; e.g. removed from pool A; and imported into another pool B.

-> the problem is that the zone has to be removed in this process and imported.
-> large zone imports (20-30k records) can take a couple of hours as Designate creates all records one by one

At the point when the zone is deleted, but not imported -> any DNS queries to Designate-MDNS from the backend will not work.
The above problem can be fixed by adding support of zone move command.
This also avoids zone export as well as zone delete steps.

 openstack zone move --pool_id=target_pool_id e.g. pool B

1. Designate should update zone pool_id similar to other zone properties
2. Designate should do the same steps as during zone creation, except it should skip creating a new DB entry
e.g. designate has to trigger new zone creation on pool B backend servers (i.e. rndc)
3. The zone transfer(AXFR/IXFR) will happen and the zone on pool B gets synced with the Designate database.
4. Zone from pool A will be removed after the administrator has changed the settings manually at domain registrar. This is manual step.