scheduler registry logs ERROR for TASK_KILLED

[jdef]

this creates pretty noisy scheduler log files. a level of INFO is probably just fine.

evidence:

E0212 22:52:01.545157    9448 registry.go:324] task killed: &TaskStatus{TaskId:&TaskID{Value:*pod.ab9fd43c-d1da-11e5-8a9a-525400309a8f,XXX_unrecognized:[],},State:*TASK_KILLED,Data:nil,Message:*pod-deleted,SlaveId:&SlaveID{Value:*20150628-154106-117441034-5050-1315-S2,XXX_unrecognized:[],},Timestamp:*1.455317518e+09,ExecutorId:&ExecutorID{Value:*31df9d040f057abd_k8sm-executor,XXX_unrecognized:[],},Healthy:nil,Source:*SOURCE_EXECUTOR,Reason:nil,Uuid:*[56 239 183 87 209 219 17 229 158 224 82 84 0 231 66 70],Labels:nil,ContainerStatus:&ContainerStatus{NetworkInfos:[&NetworkInfo{Protocol:nil,IpAddress:*10.2.0.6,Groups:[],Labels:nil,IpAddresses:[&NetworkInfo_IPAddress{Protocol:nil,IpAddress:*10.2.0.6,XXX_unrecognized:[],}],XXX_unrecognized:[],}],XXX_unrecognized:[],},XXX_unrecognized:[],}, task <nil>