Purge remaining heat_keystoneclient v2 code

Removes the remaining v2 specific code, and converts the trust
handling code to use only the v3 client.

This completes the keystone-v3-only blueprint.

blueprint: keystone-v3-only
Change-Id: Iac5b7dd73f5827cc5247a3b71d63a0f5ea266628
Merge "Purge remaining heat_keystoneclient v2 code"