Generic Event String Selector

[urso]

preparing for #2064, implement documented logic for customizable keys.

Differs from #2064 that a single-keyword and a multi-keyword can be configured. If both given, the single-keywork is used as default (last selector in list) if all others do fail.

e.g.

output.elasticsearch.pipelines:
- pipeline: "%{[key1]}"
- pipeline: "default"

will be equivalen to:

output.elasticsearch.pipelines:
- pipeline: "%{[key1]}"

output.elasticsearch.pipeline: "default"

On the other hand:

output.elasticsearch.pipeline: "pipeline"

is syntactic sugar for

output.elasticsearch.pipelines:
- pipeline: "pipeline"

Being generic, functionality is re-usable for es-index, logstash-index, redis-key, kafka-topic and others.

[urso]

blocked by #2083, #2085 and #2086,

[urso]

blocked by required bug-fixes in go-ucfg: elastic/go-ucfg#46 , elastic/go-ucfg#47, elastic/go-ucfg#48

[urso]

All blockers have been merged. Ready for review.

[monicasarbu]

LGTM

[tsg]

what's the numbering here? Is this supposed to be 1.?

[tsg]

Merged like this, in case the comment above is wrong, it can be easily fixed in a future PR.