* Use virtualenv to install tox in behave Dockerfile
Upstream change in postgres docker image uses debian restriction on
installing system-wide non-debian python packages. Debian doesn't
provide a tox>=4, so we need to install with pip.
* Exclude all output directories generated using `tox-wrapper.sh`
The `tox-wrapper.sh` script created by `features/Dockerfile` creates
directories like features/output-tox-pg14-docker-behave-etcd-lin-973719674/
* Reduce footprint of tox behave docker image
Expanding on the addition of docstrings in code, this adds python module API docs to sphinx documentation.
A developer can preview what this might look like by running this locally:
```
tox -m docs
```
The option `-W` is added to the tox env so that warning messages are considered errors.
Adds doc generation using the above method to the test GitHub workflow to catch documentation problems on PRs.
Some docstrings have been reformatted and fixed to satisfy errors generated with the above setup.
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
