Update opts.py to allow for 'None' timeout-value in client_options

[MoBoo]

Rally sets a default timeout (60s) for every http api call to elasticsearch.
For long running queries this timeout introduces some problems, because rally will fail with a client timeout (which is only traceable via the logs). To work around this timeout issues, one must set the timeout to an arbitrary high value to account for any possible timeouts.

Rally actually supports the following three options for values set in the --client-options parameter: int, float or boolean. For the timeout value an additional None is required to allow not specifing a timeout at all.
This is due to the underlying urllib3 which requires either an int, float or None value as a timeout as shown by the following error message: .../urllib3/util/timeout.py": ValueError: Timeout value connect was None, but it must be an int, float or None.
So far only int and float are usable from the commandline because Nonecannot be passed via the --client-options parameter.

This pull request fixes this issue by added the needed functionality to the opts.py module. A to_none() function is added which parses the String "None" to the pythonic None and is called in the kv_to_map() function which parses the --client-options parameters.

Edit: signed contributor agreement

[cla-checker-service]

💚 CLA has been signed

[pquentin]

Thanks! I like the idea. Can you please sign the CLA, document this and add a test in tests/utils/opts_test.py?

[pquentin]

Thanks for iterating so quickly! This now looks good to me.

We don't merge on Fridays to avoid breaking our nightly benchmarks (https://elasticsearch-benchmarks.elastic.co/), so we'll merge on Monday.

[b-deam]

Thanks again @MoBoo !