Make generation of systemd unit files deterministic

[soenkeliebau]

Currently the structure of systemd files that are generated is not determined by any specific set of rules, which means that the ordering of entries may change if for example the order that environment variables are specified in the pod is changed.

We need to come up with a set of rules to apply during generation of these files to ensure that only actual changes to the unit file trigger a rewrite.

Something along the lines of:

• write known sections in this order (unit, service, install)
• write rest of sections in alphabetical order
• sort lines within sections alphabetically