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:

まず既存の Vultr provider のテストを確認し、続いてリンクされている pytest と pytest-vcr のドキュメントを読んでください。この issue ではテストファイルも明確なスコープも指定されていないため、最初に Vultr のテストを書き直すべきか、補完すべきかを判断してください。完了の条件は、合意されたアプローチがあり、選択したカバレッジに対するテストスイートがパスすることです。

索引モデルが issue の本文から書いたものです。