Initial support for composable upgrades with Heat+Ansible

Initial support for composable upgrades with Heat+Ansible

This shows how we could wire in the upgrade steps using Ansible
as was previously proposed e.g in https://review.openstack.org/#/c/321416/
but it's more closely integrated with the new composable services
architecture.

It's also very similar to the approach taken by SpinalStack where
ansible snippets per-service were combined then run in a series of
steps using Ansible tags.

This patch just enables upgrade of keystone - we'll add support for
other patches in subsequent patches.

Partially-Implements: blueprint overcloud-upgrades-per-service
Change-Id: I39f5426cb9da0b40bec4a7a3a4a353f69319bdf9