Allow multiple comment-added events in gerrit trig.

In order to be able to react on multiple verdict categories (for example
Verify +1, Review +2) with one job, we need multiple definitions of
trigger-on events of the same-kind (comment-added-event). This was not
possible with the previous structure of event definitions.

The new yaml structure definition also better reflects the XML structure.

Change-Id: Idec0f2b07479c0611497126e984c13adf543ffc0
Merge "Allow multiple comment-added events in gerrit trig."