CBQE-0: added support for initial_vbucket test parameter for upgrade test in run_job_linux.sh

Change-Id: I658e80cf2df967f6e7813cdf63ac05ad75e5bee2
Reviewed-on: http://review.couchbase.org/43045
Reviewed-by: Andrei Baranouski <andrei.baranouski@gmail.com>
Reviewed-by: Sangharsh Agarwal <sangharsh.agarwal@globallogic.com>
Tested-by: Sangharsh Agarwal <sangharsh.agarwal@globallogic.com>