[AIRFLOW-2127] Keep loggers during DB migrations
[AIRFLOW-2127] Keep loggers during DB migrations

Python's logging.config.fileConfig function will,
by default,
disable all existing loggers when it is called.
The fileConfig
function is used with default arguments by Airflow
during
Alembic migrations and disables all loggers except
those from
alembic and sqlalchemy (this includes disabling
Airflow's own loggers).
This change sets the disable_existing_loggers flag
of fileConfig to
False so that it _does not_ disable any existing
loggers, allowing them
to continue working as normal.

See more in https://issues.apache.org/jira/browse/
AIRFLOW-2127.

Closes #3059 from jiffyclub/AIRFLOW-2127
/fileconfig-keep-loggers

adamhaney/airflow,malmiron/incubator-airflow,RealImpactAnalytics/airflow,adamhaney/airflow,wooga/airflow,RealImpactAnalytics/airflow,wolfier/incubator-airflow,wileeam/airflow,bolkedebruin/airflow,edgarRd/incubator-airflow,lyft/incubator-airflow,bolkedebruin/airflow,sergiohgz/incubator-airflow,DinoCow/airflow,cfei18/incubator-airflow,gtoonstra/airflow,lxneng/incubator-airflow,mistercrunch/airflow,apache/airflow,wolfier/incubator-airflow,subodhchhabra/airflow,apache/airflow,sekikn/incubator-airflow,fenglu-g/incubator-airflow,lyft/incubator-airflow,Tagar/incubator-airflow,danielvdende/incubator-airflow,bolkedebruin/airflow,mtagle/airflow,wndhydrnt/airflow,apache/airflow,Fokko/incubator-airflow,airbnb/airflow,apache/incubator-airflow,mrkm4ntr/incubator-airflow,lyft/incubator-airflow,Fokko/incubator-airflow,Acehaidrey/incubator-airflow,RealImpactAnalytics/airflow,sid88in/incubator-airflow,Acehaidrey/incubator-airflow,Acehaidrey/incubator-airflow,sid88in/incubator-airflow,mtagle/airflow,bolkedebruin/airflow,wileeam/airflow,andyxhadji/incubator-airflow,lxneng/incubator-airflow,fenglu-g/incubator-airflow,Fokko/incubator-airflow,zack3241/incubator-airflow,owlabs/incubator-airflow,dhuang/incubator-airflow,akosel/incubator-airflow,mrkm4ntr/incubator-airflow,wooga/airflow,artwr/airflow,cfei18/incubator-airflow,wileeam/airflow,danielvdende/incubator-airflow,wndhydrnt/airflow,lyft/incubator-airflow,andyxhadji/incubator-airflow,r39132/airflow,criccomini/airflow,sekikn/incubator-airflow,nathanielvarona/airflow,cfei18/incubator-airflow,dhuang/incubator-airflow,Tagar/incubator-airflow,jgao54/airflow,sid88in/incubator-airflow,zack3241/incubator-airflow,nathanielvarona/airflow,mistercrunch/airflow,apache/incubator-airflow,wileeam/airflow,wolfier/incubator-airflow,apache/incubator-airflow,sergiohgz/incubator-airflow,wndhydrnt/airflow,DinoCow/airflow,dhuang/incubator-airflow,danielvdende/incubator-airflow,CloverHealth/airflow,Tagar/incubator-airflow,owlabs/incubator-airflow,spektom/incubator-airflow,subodhchhabra/airflow,criccomini/airflow,apache/airflow,gtoonstra/airflow,malmiron/incubator-airflow,cfei18/incubator-airflow,airbnb/airflow,nathanielvarona/airflow,mistercrunch/airflow,airbnb/airflow,artwr/airflow,artwr/airflow,fenglu-g/incubator-airflow,malmiron/incubator-airflow,owlabs/incubator-airflow,sid88in/incubator-airflow,gtoonstra/airflow,nathanielvarona/airflow,akosel/incubator-airflow,DinoCow/airflow,Acehaidrey/incubator-airflow,edgarRd/incubator-airflow,mrkm4ntr/incubator-airflow,edgarRd/incubator-airflow,wndhydrnt/airflow,cfei18/incubator-airflow,bolkedebruin/airflow,yk5/incubator-airflow,adamhaney/airflow,criccomini/airflow,sergiohgz/incubator-airflow,danielvdende/incubator-airflow,owlabs/incubator-airflow,spektom/incubator-airflow,Fokko/incubator-airflow,yk5/incubator-airflow,jgao54/airflow,andyxhadji/incubator-airflow,danielvdende/incubator-airflow,fenglu-g/incubator-airflow,sekikn/incubator-airflow,mistercrunch/airflow,wooga/airflow,andyxhadji/incubator-airflow,artwr/airflow,mtagle/airflow,malmiron/incubator-airflow,criccomini/airflow,jgao54/airflow,yk5/incubator-airflow,spektom/incubator-airflow,cfei18/incubator-airflow,subodhchhabra/airflow,spektom/incubator-airflow,yk5/incubator-airflow,lxneng/incubator-airflow,akosel/incubator-airflow,r39132/airflow,nathanielvarona/airflow,gtoonstra/airflow,akosel/incubator-airflow,apache/airflow,wooga/airflow,sekikn/incubator-airflow,danielvdende/incubator-airflow,apache/incubator-airflow,mrkm4ntr/incubator-airflow,subodhchhabra/airflow,sergiohgz/incubator-airflow,CloverHealth/airflow,DinoCow/airflow,zack3241/incubator-airflow,Tagar/incubator-airflow,jgao54/airflow,apache/airflow,r39132/airflow,nathanielvarona/airflow,CloverHealth/airflow,lxneng/incubator-airflow,Acehaidrey/incubator-airflow,CloverHealth/airflow,zack3241/incubator-airflow,mtagle/airflow,dhuang/incubator-airflow,r39132/airflow,airbnb/airflow,edgarRd/incubator-airflow,adamhaney/airflow,wolfier/incubator-airflow,Acehaidrey/incubator-airflow,RealImpactAnalytics/airflow