Update demo directory to yaml
Make hosts.yml optional.  Load DTD files on demand.

Trying to reduce the number of magic files required to get an etch
server running.  For some historical reason I don't even remember at
this point hosts.xml was required to exist even if it was basically
empty.  I no longer see a reason to require that.  And the DTD files are
no longer needed if you have a pure YAML configuration, so load them
only if we encounter a XML config file.