add policy entries for /v3/regions

This adds entries to policy.json for v3 region CRUD including a refactor
around the create_region* methods such that there only needs to be one
entry in policy.json for both API methods (POST /v3/regions and PUT
/v3/regions/{region_id} are both enforced by 'identity:create_region').

This also corrects the HTTP response code on create_region (from 200 OK
to 201 Created) by explicitly setting it and makes the tests a bit more
dynamic.

Closes-Bug: 1272496
Closes-Bug: 1272501
Change-Id: Icaa9b005788b50342d5fdb334055f5fad25436d9
Merge "add policy entries for /v3/regions"