Fix triggers for release-approval pipeline

The release-approval pipeline implemented a nice infinite loop as
it was triggered by all comment-added events, and Zuul is posting one
every time the job runs.

Add a regexp to exclude comment-added events from the 'zuul' user.

Also since patchset-created actually does not trigger comment-added,
it needs to be specified separately (in order to approve releases
proposed by the PTL themselves).

Change-Id: I0f8b8526adaa83876ff92a98500d35c70cc23174

zuul.d/pipelines.yaml

@@ -207,7 +207,9 @@
         current-patchset: True
     trigger:
       gerrit:
+        - event: patchset-created
         - event: comment-added
+          username: ^(?!zuul\b)\b\w+$
     success:
       gerrit:
         PTL-Approved: 1