Make S3 support optional

[pquentin]

Closes #1095.

With this pull request, boto3 is no longer installed by default but should be installed using python -m pip install esrally[s3] instead.

When a S3 base-url is provided, here's what users see:

[INFO] Preparing for race ...
[ERROR] S3 support is optional. Install it with `python -m pip install esrally[s3]`

[ERROR] Cannot race. Error in track preparator
        No module named 'boto3'

...

So, we let the ImportError propagate, but also explain how to get S3 support.

To test this manually in an existing virtual environment, uninstalling boto3 manually is not enough, as pkg_resource.require("esrally") will complain that it's missing. Recreating the virtual environment from scratch and removing any .egg-info files should fix this.

For what it's worth, I haven't managed to get the integration tests to work locally, but they don't seem to use S3, so they probably still pass.

(This is only about S3 as suggested by #1095, but I'll be happy to do the same thing for GCS if this PR gets merged.)

[pquentin]

This is quite ugly, but the alternative is to use del sys.modules["boto3"], which is an ugly and global hack, and likely to break things. While we use this hack succesfully in urllib3 to test support for Python versions without ssl, this:

• has caused huge headaches in a project that adds async support to urllib3 and
• would prevent running tests in parallel.

So I would suggest avoiding touching sys.modules.

[DJRickyB]

@elasticmachine run tests please

[pquentin]

Thanks for running the tests! I'll fix the lint

[pquentin]

@DJRickyB Can you please run the tests again? I guess @elasticmachine won't listen if I try to ask myself

[DJRickyB]

@elasticmachine run tests

[DJRickyB]

@pquentin thanks a ton! I have one remaining requirement before approval: please preserve the behavior where make install ends up installing boto3+dependencies. for our purposes I think it's ok if [s3] resolution occurs in either make install-user or a new make target altogether

[pquentin]

@DJRickyB Thank you for the review! Done

[DJRickyB]

Looks good to me. Thank you once again for your contribution

[pquentin]

Thank you for the quick reviews! I'll work on making GCS support optional next.

[DJRickyB]

@pquentin thank you, but we are looking at keeping GCS support first-class for the time being