Clean up env var when returning cells
Clean up env var when returning cells

ONOS_USER, ONOS_GROUP, ONOS_WEB_USER and ONOS_WEB_PASS will be reset to default value when cleaned up.
Users can override the default value by specifying desired value in local cell files.

Change-Id: I52badd0446e109fcfc7ff94779ef228e8d8276f9

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