Overhaul server configuration

* switch back to using the nconf module
* enable command line config option overrides
* change config language from JSON to YAML for readibility
* flattened config file format and removed 'settings' sub-object
* renamed many config fiels
* new nconf-style default values system in lib/common/default-config.js
* changed validator modules from JSV/jsvutil to revalidator
  (much more popular and better maintained, same functionality)
* cleaned up schema
* changed overseer server settings to a base URL instead of separate
  host & port, stop using svmp-server's ssl enable to set use of ssl
  on overseer API calls

Change-Id: I0012a5b385feedb7a6e856bbc9c2fb851eeb68ca