Implement Python helpers for using RNDC in tests

Controlling named instances using RNDC is a common action in BIND 9 system tests. However, there is currently no standardized way of doing that from Python-based system tests, which leads to code duplication. Add a set of Python classes and pytest fixtures which intend to simplify and standardize use of RNDC in Python-based system tests.

For now, RNDC commands are sent to servers by invoking the rndc binary. However, a switch to a native Python module able to send RNDC commands without executing external binaries is expected to happen soon. Even when that happens, though, having the capability to invoke the rndc binary (in order to test it) will remain useful. Define a common Python interface that such "RNDC executors" should implement (RNDCExecutor), in order to make switching between them convenient.