Add proposed /regions resource to Keystone 3.2 API

This patch adds initial specification for the proposed
regions resource collection in the v3.2 Keystone API.

The regions resource allows for a deployer to describe
a hierarchical layout of their entire deployment, with
regions having an optional parent region.

partially implements bp first-class-regions

Change-Id: Ic49c0aeb2bec7327699d9c7e72df556d78b20e32
Merge "Add proposed /regions resource to Keystone 3.2 API"