FlowETL config should always be validated when reading from a config file

[maxalbert]

Currently the FlowETL config is validated (and default values filled in) in dags/etl.py after it is read from the config.yml file.

However, get_config_from_file () is also used in another place without subsequent calls to validate_config() and fill_config_default_values(). This can cause strange errors in production if the config file isn't valid or is missing default values.

Therefore those subsequent validation/filling steps should happen in get_config_from_file() directly.