LU-1798 tests: don't set jobid if not changing

Don't set jobid_var conf_param if the value would not change.  This
avoids setting the same parameter in the config log multiple times,
and avoids the extra delay on each startup waiting for the conf_param
to propagate from the MGS to the client each time.

Fix the console error printing if the jobid_env does not exist.

Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: I36ed60451a875bcd46fd0d6f3d7068d1b1398df5
Reviewed-on: http://review.whamcloud.com/3867
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Niu Yawei <yawei.niu@intel.com>
Reviewed-by: Jian Yu <jian.yu@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>