Missing manual for end-to-end third party integration

[Emilien Macchi]

As a third party service of TripleO (e.g. anything that is not supported in TripleO, like Dell Powerflex services, or any OpenStack service not supported by us), they can be deployed with out-of-tree files. Some bits are documented in tripleo-docs already but there is no step-by-step guide which explains it.

We could have a new guide that explain the step by step approach and when possible, provide links if something is documented already.

e.g. the container build part is documented here:

We need to document:

- the composable services part (out of tree templates); which links to https://docs.openstack.org/tripleo-docs/latest/developer/tht_walkthrough/tht_walkthrough.html (and by the way this manual needs love too, lot of things are outdated).

- the Ansible role best practices (if the service is deployed by Ansible) and provide examples with tripleo-ansible roles.

- If the service is containerized, explain how to build container images (with TCIB, https://docs.openstack.org/project-deploy-guide/tripleo-docs/latest/deployment/3rd_party.html)

- Third party testing, pretty much everything is documented here: https://docs.openstack.org/tripleo-docs/latest/ci/third_party_dependencies_ci.html but explain it in the guide how it's possible.