Provide function to execute when calling sls wsgi command

[estahn]

We run django in AWS Lambda and want to apply database migrations via sls wsgi manage. The database user the application runs with is limited and not allowed to create/alter tables. Thus we want to define an alternate Lambda function with different environment variables providing database admin credentials. This is primarily a security concern to segregate the database base user the application uses from the database user the migration script uses.

findHandler currently returns the first function with condition handler: wsgi_handler.handler.

https://github.com/logandk/serverless-wsgi/blob/master/index.js#L436C7-L440

Instead of returning the first function, if --function / -f (required) is specified, the specified function should be used.