Tripleo: exploit ansible to customize both the Undercloud and the Overcloud

<!--HTML-->TripleO, <https://docs.openstack.org/tripleo-docs/latest/>, is a set of tools for the deployment and management of OpenStack. Its strategy consists in using a underlying OpenStack installation (undercloud) to install and manage the main one (overcloud). It's the installation method used by RDO, <https://www.rdoproject.org/>. In our project to deploy a HyperConverged (HCI) OpenStack cloud infrastructure, we wrote some ansible roles to prepare the undercloud and setup the templates required to configure the overcloud. We also added some additional steps, to fully support OIDC authentication in keystone and to add external prometheus metrics to the physical nodes.