Small mistake in grammar.treetop (Logstash config parser)

There is a rule for `array_value` in the treetop parser definition (https://github.com/elastic/logstash/blob/master/logstash-core/lib/logstash/config/grammar.treetop#L66-L68), which is never used. So I assume that either this definition could be removed or maybe more likely this rule should be used for values in arrays on https://github.com/elastic/logstash/blob/master/logstash-core/lib/logstash/config/grammar.treetop#L100 as I guess, that a `plugin` is not a valid value within an array (this is the difference between the rules for `value` and `array_value`).

@jordansissel I guess you should have a look at this.

If you wish, I could provide a PR.