refactor(*): rework cloudconfig for better extensibility and consistency

This change creates a few simple interfaces for coreos-specific
configuration options and moves things to them wherever possible; so if
an option needs to write a file, or create a unit, it is acted on
exactly the same way as every other file/unit that needs to be written
during the cloud configuration process.
refactor(systemd): don't allow users to set DropIn=true yet
Merge pull request #100 from jonboulle/rework

refactor(*): rework cloudconfig for better extensibility and consistency