Refactor the agent code

[soenkeliebau]

During the early life of the agent some decisions on code structure were not taken the way that they maybe shoud have taken.

This makes the code less testable and in places more unstable as well as hampering future changes.

We should take some time to have a close look at the agent and resolve some of these issues as early as possible, before they impact too much of the follow up development in a negative way.

We won't structure the work completely up front, @siegfriedweber will create issues under this epic as and when needed, when he gets around to looking at this.

Issues:

• Move file format handling out of systemdunit.rs #244