Health report: use device pk and not __unicode__
Health report: use device pk and not __unicode__

Change-Id: I55aaebb2ae1ac35733d799933d7717507a7c9260

OSSystems/lava-server,OSSystems/lava-server,Linaro/lava-server,Linaro/lava-server,Linaro/lava-server,OSSystems/lava-server,Linaro/lava-server