Improve support Elasticsearch 5 as metrics store

[danielmitterdorfer]

While Rally's mapping for the metrics store currently works also with ES 5 due to backwards compatibility features (string / not_analyzed is automatically converted to keyword), we should natively support it as this feature in ES will likely be removed at some point.

[jpountz]

I'm curious which API break is responsible for the fact that it does not work with 5.0?

[danielmitterdorfer]

@jpountz I worried about the use of string/ not_analyzed instead of keyword. I've just tried it with ES 5.0 alpha3 and it works due to a fallback that auto-converts these fields automatically to keyword. So I'll relabel it as an enhancement because this auto-conversion will be removed at some point in ES.

[jpountz]

Thanks! The auto conversion should be removed in 6.0 indeed.

[danielmitterdorfer]

As we'll change the metrics store structure in #279 anyway and Elasticsearch 5.0 is now out for quite some time, we should target this for the next release.