When the playbook executes on a remote host (as it usually does),
we need to explicitly say `src` is a remote file and not relative
to the playbook.

(adapting strings and URLs)

Discussed in #101 (Github Issue).

Fixes #427 (Github Issue).

Related to #425 (Github Pull Request)

Fixes #424 (Github Issue).

Fixes #417 (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).

Some options are no longer required and have sensible default values.