Fix cloud plugins to have real mocks for tests

[clintongormley]

• These only have shallow tests and are fragile. we rely on users to report bugs
• need disco test in qa/ with mock ec2
• need snapshot test in qa/ with mock s3
• ideally use proxy too for testing that

[clintongormley]

@jasontedor please could you try to tackle this one

[dadoonet]

@clintongormley @jasontedor FYI I'm currently working on ec2 mock

[s1monw]

@dadoonet can you sync with @jasontedor on this please

[dadoonet]

For the record, @ywelsch just added some new tests for EC2 with #17677
@s1monw already added tests for GCE with #16881

[williamrandolph]

I believe this work has been handled in #31107 and #29315.