add ability to specify a external journal for each target.

Signed-off-by: Alexey Lyashkov <alexey.lyashkov@seagate.com>
Change-Id: I41005d3b78935185b3498513691fc250e9c57fe2
Reviewed-on: http://review.whamcloud.com/14426


Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>