add arp cmd; always return lists from checkLog
add arp cmd; always return lists from checkLog

The arp CLI command sends gratuitous ARPs from all hosts - this
makes it easier for ONOS to discover hosts.

Previously checkLog returned None, None if the log file didn't
exist; now we return an empty list, to avoid breaking when we
take the len() of errors or warnings (which was happening before)
and to avoid having to check for None, None, which didn't seem to
add a lot of value to justify its complexity.

Change-Id: I84847fcf70525c82ac037d8a84cf40d21ab2a106

LorenzReinhart/ONOSnew,VinodKumarS-Huawei/ietf96yang,VinodKumarS-Huawei/ietf96yang,sdnwiselab/onos,Shashikanth-Huawei/bmp,oplinkoms/onos,gkatsikas/onos,opennetworkinglab/onos,gkatsikas/onos,VinodKumarS-Huawei/ietf96yang,mengmoya/onos,maheshraju-Huawei/actn,kuujo/onos,kuujo/onos,kuujo/onos,LorenzReinhart/ONOSnew,donNewtonAlpha/onos,oplinkoms/onos,maheshraju-Huawei/actn,kuujo/onos,mengmoya/onos,maheshraju-Huawei/actn,kuujo/onos,Shashikanth-Huawei/bmp,oplinkoms/onos,gkatsikas/onos,kuujo/onos,LorenzReinhart/ONOSnew,opennetworkinglab/onos,maheshraju-Huawei/actn,gkatsikas/onos,y-higuchi/onos,donNewtonAlpha/onos,osinstom/onos,y-higuchi/onos,VinodKumarS-Huawei/ietf96yang,osinstom/onos,opennetworkinglab/onos,donNewtonAlpha/onos,osinstom/onos,VinodKumarS-Huawei/ietf96yang,maheshraju-Huawei/actn,Shashikanth-Huawei/bmp,sdnwiselab/onos,gkatsikas/onos,sdnwiselab/onos,opennetworkinglab/onos,osinstom/onos,mengmoya/onos,mengmoya/onos,LorenzReinhart/ONOSnew,sdnwiselab/onos,oplinkoms/onos,mengmoya/onos,donNewtonAlpha/onos,opennetworkinglab/onos,y-higuchi/onos,y-higuchi/onos,y-higuchi/onos,oplinkoms/onos,oplinkoms/onos,LorenzReinhart/ONOSnew,Shashikanth-Huawei/bmp,opennetworkinglab/onos,kuujo/onos,gkatsikas/onos,sdnwiselab/onos,sdnwiselab/onos,Shashikanth-Huawei/bmp,oplinkoms/onos,donNewtonAlpha/onos,osinstom/onos