Suggested refactor ptfhttpxmlrestvalue sensor

Raised by @lucianolingnau

One of the templates created by this script deploys several ptfhttpxmlrestvalue sensors. There's a new "Rest Custom" sensor that can monitor multiple XML/JSON nodes via a single sensor. Some examples:

https://kb.paessler.com/en/topic/75798-rest-custom-sensor-wunderground
https://kb.paessler.com/en/topic/78049-rest-custom-sensor-example-for-bitcoin-via-coindesk