* Allow the mautrix whatsapp relaybot to be enable with a variable
This allows a user to enable the relaybot by setting a variable in
`vars.yml` in the same way that the mautrix signal relaybot is
configured.
* Correct default values for mautrix whatsapp relaybot variables
* Add documentation for using the relaybot with mautrix whatsapp
* Adjust variable names to better reflect what they do
* Set default variables properly and use to_json in template
- forego removing Docker images - it's not effective anyway, because it
only removes the last version.. which is a drop in the bucket, usually
- do not reload systemd - it's none of our business. `--tags=start`,
etc., handle this
- combine all uninstall tasks under a single block, which only runs if
we detect traces (a leftover systemd .service file) of the component.
If no such .service is detected, we skip them all. This may lead to
incorect cleanup in rare cases, but is good enough for the most part.
This was useful when the order of these roles in relation to Synapse
mattered (when we were injecting stuff into Synapse variables during
runtime). This is no longer the case since 0ea7cb5d18, so all of
this can be removed.
