Related to #643 (Github Issue)

Changelog here https://github.com/jitsi/jitsi-meet/releases/tag/stable%2Fjitsi-meet_4857

Related to #554 (Github Pull Request).

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.

Related to #508 (Github Pull Request).

No migration steps should be required.

Hopefully fixes an error like this (which I haven't been able to
reproduce, but..):

> [modules/xmpp/strophe.util.js] <Object.i.Strophe.log>:  Strophe: Error: Failed to construct 'RTCPeerConnection': 'matrix.DOMAIN' is not one of the supported URL schemes 'stun', 'turn' or 'turns'.

These are some new defaults that we switch to, to follow upstream
changes:

- https://github.com/jitsi/docker-jitsi-meet/commit/264df04
- 3121494d4b

We define this password in the `sip-communicator.properties`
configuration file, so this is not needed for actually running JVB.

However, it does a (useless) safety check during container startup,
and we need to make that check happy.

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).