add alternative tags on grokparsefailure

Sometimes adding "_grokparesfailre" is not what we want, we want one or more alternative tag names for more fine grained filtering control in the pipeline.
"_grokparesfailre" is still the default.

<pre>
filter {
  grok {
    tags                => [ "apache-accesslog" ]
    patterns_dir        => [ "/opt/logstash/patterns" ]
    match               => [ "bytes", "%{IP}" ]
    break_on_match      => true
    named_captures_only => true
    tag_on_failure      => [ "_bad_clientip" ]
  }
}
</pre>