Better integration with systemd

**Description**

Airflow services (scheduler, worker and webserver) can be managed with systemd. Some small improvements will make this experience better:
- systemd has a feature called [WatchDogSec](https://www.freedesktop.org/software/systemd/man/systemd.service.html). It can be used to restart hung processes. To make use of the feature, the service needs to periodically call `sd_notify(3)`. It'll be nice to let the `heartbeat()` function call this so that systemd can monitor Airflow services.
- Airflow services by default writes a pid file to `AIRFLOW_HOME`. If the `AIRFLOW_HOME` is in a distributed file system, it causes some conflicts. E.g. when there are multiple airflow-worker or airflow-webserver being managed by systemd on different hosts. systemd already makes sure there's only one instance of the service running. So the pid file is redundant. It'll be nice to be able to turn off the pid file when systemd is used.