Fix liveness check

[blogh]

The liveness probe used to return something. It looks like it doesn't do it anymore and it breaks the node_is_alive check.

Issue: #31

[blogh]

The service now returns:

(.venv) [benoit@benoit-dalibo check_patroni]$ check_patroni --config config.ini -vvv node_is_alive
DEBUG - Trying to connect to http://10.20.30.51:8008/liveness with cert: None verify: None
DEBUG - Starting new HTTP connection (1): 10.20.30.51:8008
DEBUG - http://10.20.30.51:8008 "GET /liveness HTTP/1.1" 200 None
DEBUG - api call data: <Empty>
NODEISALIVE OK - This node is alive (patroni is running).
| is_alive=1;;@0

Instead of crashing as descrbed in the issue.

[blogh]

@dlax can you have a quick look at this? I am not sure the fix is very "pythonic" ?