Fix cells command
Fix cells command

"cells" throws errors when $ONOS_CELL is not set

Change-Id: Ia41bbd9e877dbffc86ddaaf0f5e5adcd96bba2ad

sdnwiselab/onos,LorenzReinhart/ONOSnew,osinstom/onos,sdnwiselab/onos,sdnwiselab/onos,kuujo/onos,gkatsikas/onos,opennetworkinglab/onos,sdnwiselab/onos,oplinkoms/onos,kuujo/onos,kuujo/onos,kuujo/onos,LorenzReinhart/ONOSnew,LorenzReinhart/ONOSnew,LorenzReinhart/ONOSnew,oplinkoms/onos,osinstom/onos,opennetworkinglab/onos,kuujo/onos,kuujo/onos,sdnwiselab/onos,kuujo/onos,osinstom/onos,gkatsikas/onos,osinstom/onos,LorenzReinhart/ONOSnew,oplinkoms/onos,sdnwiselab/onos,oplinkoms/onos,oplinkoms/onos,gkatsikas/onos,gkatsikas/onos,opennetworkinglab/onos,oplinkoms/onos,oplinkoms/onos,opennetworkinglab/onos,gkatsikas/onos,gkatsikas/onos,osinstom/onos,opennetworkinglab/onos,opennetworkinglab/onos