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.
The update_engine parsing and XML generation code is very naive. Instead
of trying to implement a correct parser and generater in C++ just
generate a file that doesn't have quote's around fields that we know
won't have spaces.
