Delete unused config

This version of gogeta-update is minimally feature-complete.

It takes a configuration file and can:

 - list the running configuration
 - change the running configuration
 - delete unused services
 - operate in dry-run mode to show what would change
Make the input config match output

Redo the YAML files based on the output of yaml.dump so that it's
possible to diff one configuration against the other.
Add a purge command

This option will completely destroy the gogeta config (which can be
combined with an update) in case you need to recover from serious
errors.