Standardize gerrit_[merge|verify]_triggers

It turns out you can override per job definition inside of a projects
declaration so let's standardize the trigger wording so that they are
the same everywhere.

Change-Id: Id489a3b04242183a35d56bb938c66b4b486a1d1f
Signed-off-by: Thanh Ha <thanh.ha@linuxfoundation.org>