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.
Fix#69 - A user may provide an `enable` attribute of a unit in their
cloud config document. If true, coreos-cloudinit will instruct systemd
to enable the associated unit. If false, the unit will not be enabled.
Fix#71 - The default enable behavior has been changed from on to off.
