feature request, improving tests with pytest and pytest-vcr

## Feature Permission Request

Hi, there. Please keep in mind:
- my English is very poor
- I have a very limited experience with any open source projects, especially this one

Recently I'v made a small changes to some cloud providers:
- https://github.com/apache/libcloud/pull/1508 (Digitalocean)
- https://github.com/apache/libcloud/pull/1506 (Vultr)

I am asking for permission for completely rewrite some tests with:
- https://docs.pytest.org/en/stable/
- https://pytest-vcr.readthedocs.io/en/latest/

I chose Vultr for a start point for me.
I believe that `pytest` is a very nice and easiest way to write any tests for any project. Many people do not like write tests, but with `pytest` and `pytest-vcr` their work will be more funny and much better.
I can not promise that my work will be done completely, but I will try to make a start point for that.

Please suggest me the write way. E.g.:
- I should not make any changes to current tests
- I should make new tests from the scratch

P.S.
I hope Google translate is working as expected :smiley:

Commencez par examiner les tests existants du provider Vultr, puis consultez la documentation liée de pytest et pytest-vcr. L’issue n’identifie aucun fichier de test ni périmètre défini ; déterminez donc d’abord si les tests Vultr doivent être réécrits ou complétés. La tâche sera considérée comme terminée lorsqu’une approche aura été convenue et que la suite de tests passera pour la couverture sélectionnée.

Rédigé par le modèle d'indexation à partir du texte de l'issue.