Draft: Add Python helpers for RNDC and other instance specific functions for system tests

This builds on Michał's work implementing rndc as Pytest fixtures.

Things I added:

• querying a instance (a replacement for calling out to dig and grepping its output)
• templating configuration and reloading a instance with it (replacement for copy_setports, I have to set up the jinja2 dependency properly though)

I then rewrote partially the emptyzones test using these. Partially, because I failed to find a way to hook into the current pytest machinery in such a way that would allow me to remove the initial templating in setup.sh.