Depending on the distro, common commands like sleep and chown may either
be located in /bin or /usr/bin.

Systemd added path lookup to ExecStart in v239, allowing only the
command name to be put in unit files and not the full path as
historically required. At least Ubuntu 18.04 LTS is however still on
v237 so we should maintain portability for a while longer.

Fixes #427 (Github Issue).

We do this for 2 reasons:

- so we can control things which are not controllable using environment
variables (for example `stunServers` in jitsi/web, since we don't wish
to use the hardcoded Google STUN servers if our own Coturn is enabled)

- so playbook variable changes will properly rebuild the configuration.
When using Jitsi environment variables, the configuration is only built
once (the first time) and never rebuilt again. This is not the
consistent with the rest of the playbook and with how Ansible operates.
We're not perfect at it (yet), because we still let the Jitsi containers
generate some files on their own, but we are closer and it should be
good enough for most things.

Related to #415 (Github Pull Request).