[AIRFLOW-2809] Fix security issue regarding Flask SECRET_KEY
[AIRFLOW-2809] Fix security issue regarding Flask SECRET_KEY

It's recommended by Falsk community to use random
SECRET_KEY for security reason.

However, in Airflow there is a default value for
secret_key and most users will ignore to change
it.

This may cause security concern.

Closes #3651 from XD-DENG/patch-2

mrkm4ntr/incubator-airflow,Acehaidrey/incubator-airflow,apache/incubator-airflow,wooga/airflow,Acehaidrey/incubator-airflow,akosel/incubator-airflow,wooga/airflow,malmiron/incubator-airflow,gtoonstra/airflow,Fokko/incubator-airflow,cfei18/incubator-airflow,apache/incubator-airflow,akosel/incubator-airflow,edgarRd/incubator-airflow,jgao54/airflow,fenglu-g/incubator-airflow,spektom/incubator-airflow,mrkm4ntr/incubator-airflow,jgao54/airflow,Acehaidrey/incubator-airflow,lyft/incubator-airflow,mistercrunch/airflow,mrkm4ntr/incubator-airflow,mtagle/airflow,adamhaney/airflow,danielvdende/incubator-airflow,sekikn/incubator-airflow,adamhaney/airflow,apache/airflow,nathanielvarona/airflow,mtagle/airflow,airbnb/airflow,danielvdende/incubator-airflow,mistercrunch/airflow,apache/airflow,akosel/incubator-airflow,mistercrunch/airflow,cfei18/incubator-airflow,mistercrunch/airflow,Fokko/incubator-airflow,cfei18/incubator-airflow,DinoCow/airflow,artwr/airflow,wileeam/airflow,fenglu-g/incubator-airflow,mtagle/airflow,dhuang/incubator-airflow,edgarRd/incubator-airflow,danielvdende/incubator-airflow,sid88in/incubator-airflow,DinoCow/airflow,adamhaney/airflow,cfei18/incubator-airflow,artwr/airflow,Fokko/incubator-airflow,mtagle/airflow,criccomini/airflow,wileeam/airflow,bolkedebruin/airflow,gtoonstra/airflow,edgarRd/incubator-airflow,criccomini/airflow,fenglu-g/incubator-airflow,andyxhadji/incubator-airflow,apache/incubator-airflow,sekikn/incubator-airflow,spektom/incubator-airflow,akosel/incubator-airflow,apache/incubator-airflow,airbnb/airflow,cfei18/incubator-airflow,nathanielvarona/airflow,sid88in/incubator-airflow,bolkedebruin/airflow,airbnb/airflow,apache/airflow,nathanielvarona/airflow,dhuang/incubator-airflow,mrkm4ntr/incubator-airflow,lyft/incubator-airflow,malmiron/incubator-airflow,criccomini/airflow,sid88in/incubator-airflow,sekikn/incubator-airflow,fenglu-g/incubator-airflow,malmiron/incubator-airflow,adamhaney/airflow,wileeam/airflow,spektom/incubator-airflow,r39132/airflow,malmiron/incubator-airflow,danielvdende/incubator-airflow,wooga/airflow,andyxhadji/incubator-airflow,cfei18/incubator-airflow,lyft/incubator-airflow,dhuang/incubator-airflow,edgarRd/incubator-airflow,jgao54/airflow,DinoCow/airflow,airbnb/airflow,criccomini/airflow,lyft/incubator-airflow,gtoonstra/airflow,r39132/airflow,andyxhadji/incubator-airflow,DinoCow/airflow,wileeam/airflow,nathanielvarona/airflow,apache/airflow,spektom/incubator-airflow,nathanielvarona/airflow,danielvdende/incubator-airflow,artwr/airflow,jgao54/airflow,bolkedebruin/airflow,apache/airflow,bolkedebruin/airflow,sid88in/incubator-airflow,gtoonstra/airflow,r39132/airflow,Fokko/incubator-airflow,danielvdende/incubator-airflow,sekikn/incubator-airflow,dhuang/incubator-airflow,apache/airflow,andyxhadji/incubator-airflow,bolkedebruin/airflow,artwr/airflow,Acehaidrey/incubator-airflow,Acehaidrey/incubator-airflow,wooga/airflow,Acehaidrey/incubator-airflow,nathanielvarona/airflow,r39132/airflow