Seamless install playbook, with minimal parameters, default values or mandatory statement

In 763d458c we have a very basic version of the collection, with a running cielito.sympa.install roles which does the job just with a call:

roles:
  role: cielito.sympa.install

Or almost: it requires a non obvious set of variables for all the roles called by the playbook, complexity that is still hidden in our IaC, not shared in this collection. So we should bring it here.

There is no simple way (if there is any) to set default values in a collection playbook. (there is no easy way neither to set paths relative to the original calling script, not the collection playbook). So let's try to wrap in a collection's role, let's say cielito.sympa.infra, the definition of these "Sympa standard" default values of roles postgresql, nginx, certbot and postfix, as well as the execution call of the roles with appropriate variables' values.