Fix elasticsearch metrics according to different ES versions

Tested with ES 0.90.5 -> 0.90.10.
This is related to https://github.com/DataDog/dd-agent/pull/1010
The new JVM metric names were introduced in ES 0.90.10. The other metrics are available starting with ES 0.90.5.
This seems weird, for version >= [0,90,10] it will override additional_metrics defined on line 188, right ?