In some circumstances (e.g. nova-agent-watcher) cloudconfig files will
be created where the EtcdEnvironment is an empty map, and hence != nil.
If this is the case we should not do anything at all (because the user
hasn't explicitly asked us to configure etcd). This change standardises
behaviour with the check that we already do for FleetEnvironment.
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.
