Allow setting arbitrary SUT parameters from Theodolite

We can assume that SUT parameters (e.g., configuration options of the stream processing engine can be configured via environment variables.

Currently, the settable parameters must be hard coded in Theodolite, from where they are replaced by the user input. This be made more generically. Per default, all variables are set to default, which are configured in the benchmark declarations (e.g., Kubernetes resource files). However, users can override those setting. For example, to test different commit intervals.

This issue should be considered when tackling #89 (closed)