test_vultrpy: adjust test expectation to prevent failure

[meaksh]

What does this PR do?

After Debian 10 got EOL, this test: tests/integration/cloud/clouds/test_vultrpy.py::VultrTest::test_list_images started failing in our Salt Shaker testsuite for all OSes. The tests expectations needs to be adapted.

There is no upstream PR for this, as these tests were dropped from upstream after extensions splitout.

Merge requirements satisfied?

• [ ] Changelog - https://docs.saltproject.io/en/master/topics/development/changelog.html
• [x] Tests written/updated

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Yes

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