[AIRFLOW-2903] Change default owner to `airflow` (#4151)
[AIRFLOW-2903] Change default owner to `airflow` (#4151)

This makes the default owner when one is not provided in the dag the same as in the examples and docs for consistency.
Acehaidrey/incubator-airflow,apache/airflow,artwr/airflow,Acehaidrey/incubator-airflow,wileeam/airflow,apache/incubator-airflow,nathanielvarona/airflow,airbnb/airflow,mrkm4ntr/incubator-airflow,Fokko/incubator-airflow,apache/airflow,apache/incubator-airflow,wooga/airflow,DinoCow/airflow,sekikn/incubator-airflow,wooga/airflow,DinoCow/airflow,r39132/airflow,wooga/airflow,r39132/airflow,apache/airflow,Acehaidrey/incubator-airflow,lyft/incubator-airflow,artwr/airflow,sekikn/incubator-airflow,lyft/incubator-airflow,adamhaney/airflow,jgao54/airflow,mrkm4ntr/incubator-airflow,apache/incubator-airflow,adamhaney/airflow,fenglu-g/incubator-airflow,Acehaidrey/incubator-airflow,adamhaney/airflow,r39132/airflow,lyft/incubator-airflow,bolkedebruin/airflow,airbnb/airflow,spektom/incubator-airflow,spektom/incubator-airflow,artwr/airflow,mistercrunch/airflow,fenglu-g/incubator-airflow,spektom/incubator-airflow,Fokko/incubator-airflow,danielvdende/incubator-airflow,jgao54/airflow,apache/airflow,mrkm4ntr/incubator-airflow,sekikn/incubator-airflow,danielvdende/incubator-airflow,DinoCow/airflow,Fokko/incubator-airflow,nathanielvarona/airflow,Fokko/incubator-airflow,cfei18/incubator-airflow,mistercrunch/airflow,Acehaidrey/incubator-airflow,malmiron/incubator-airflow,nathanielvarona/airflow,bolkedebruin/airflow,nathanielvarona/airflow,mtagle/airflow,dhuang/incubator-airflow,jgao54/airflow,wileeam/airflow,dhuang/incubator-airflow,DinoCow/airflow,mtagle/airflow,wileeam/airflow,malmiron/incubator-airflow,fenglu-g/incubator-airflow,malmiron/incubator-airflow,dhuang/incubator-airflow,cfei18/incubator-airflow,jgao54/airflow,spektom/incubator-airflow,artwr/airflow,bolkedebruin/airflow,adamhaney/airflow,nathanielvarona/airflow,apache/incubator-airflow,bolkedebruin/airflow,danielvdende/incubator-airflow,danielvdende/incubator-airflow,airbnb/airflow,apache/airflow,mistercrunch/airflow,dhuang/incubator-airflow,mistercrunch/airflow,malmiron/incubator-airflow,danielvdende/incubator-airflow,mrkm4ntr/incubator-airflow,apache/airflow,cfei18/incubator-airflow,cfei18/incubator-airflow,r39132/airflow,lyft/incubator-airflow,fenglu-g/incubator-airflow,mtagle/airflow,wileeam/airflow,wooga/airflow,danielvdende/incubator-airflow,cfei18/incubator-airflow,cfei18/incubator-airflow,nathanielvarona/airflow,mtagle/airflow,Acehaidrey/incubator-airflow,sekikn/incubator-airflow,bolkedebruin/airflow,airbnb/airflow