All users to specify API key in a place other than a header

[Benjamin Trent]

Right now to use an API key in the auth header I have to do some hoop jumping like:

esrally race --track=geonames --pipeline=benchmark-only --client-options='{"default": {"use_ssl": true, "headers":{"Authorization":"ApiKey aE...=="}}}' --target-hosts=target.es.host:9200 --kill-running-processes --test-mode

This is not well documented and has the added disadvantage of logging the headers (issue: #1757 is aimed to fix that part of it).

But, I want to be able to simply say --client-options="auth_api_key:'aE...=='" and rally does the right thing.