Use GERRIT_BRANCH instead of GERRIT_REFSPEC for merge job

- Change GERRIT_REFSPEC to GERRIT_BRANCH for builder-merge job
- Change origin/$branch to **/$branch for SCM

Change-Id: Id5437a3641b0f97cbaa9ddab6ee0293165d23426
Signed-off-by: Thanh Ha <thanh.ha@linuxfoundation.org>