Rework testplan template

* Add a test plan YAML file. It's not clear whether this file will
  remain here on the long term, but for now it was generated based on
  the current CI tests done with our OE jobs.
* Removed the ##VARS## processing, and associated Makefiles. It might
  not be needed, since we can use the YAML file instead for variables.

Signed-off-by: Nicolas Dechesne <nicolas.dechesne@linaro.org>