It gives users the option to send Patroni application logs to a File instead of Standard Out. There are three environment variables that can be set to enable and configure file logging.
1. `PATRONI_FILE_LOG_DIR`: Path to a directory that is writeable by the executing user. Having this variable set is what activates file logging.
2. `PATRONI_FILE_LOG_NUM`: This is a rolling file logger. This variable dictates how many log files are retained.
3. `PATRONI_FILE_LOG_SIZE`: This variable dictates the size at which the logs will roll.
If `PATRONI_FILE_LOG_DIR` is not set than Patroni will log to stderr (default behavior does not change)
Closes https://github.com/zalando/patroni/issues/902
