use s3ninja for custom provider testing

[sa7mon]

Vultr objects is just too flaky to depend on for automated testing. Fire up an S3 Ninja container, programmatically create some test buckets, and use those for testing.

[sa7mon]

s3 ninja is not fully featured. gofakes3 is close but doesn't support bucket permissions at all. We may just need to use httptest and assert the HTTP calls look correct. Current work in branch 174