Format user names in created bugzilla comments

As AccountAttribute does not provide a String conversion method, the
generated bugzilla comments ended in:
  ... [by com.google.gerrit.server.events.AccountAttribute@5b9b8858]
Now they end in:
  ... [by John Doe]

Change-Id: I702b83ba14ec0fd14471420e461de4d26c80baf7