Add default refspec reference

Missed this when we ported over the infra-parameters. Refspec should
default to refs/heads/{branch} so that a manual run of a job will always
work against the specified branch for verify jobs. Merge jobs ignore
this by defining a blank refspec.

Change-Id: I7d7323b85b61639eef0e1b4491d6ba94d934b714
Signed-off-by: Thanh Ha <thanh.ha@linuxfoundation.org>