* Start moving toward a json buildcache index

* Add spec and database index schemas

* Add a schema for buildcache spec.yaml files

* Provide a mode for database class to generate buildcache index

* Update db and ci tests to validate object w/ new schema

* Remove unused temporary upload-s3 command

* Use database class to generate buildcache index

* Do not generate index with each buildcache creation

* Make buildcache index mode into a couple of constructor args to Database class

* Use keyword args for  _createtarball 

* Parse new json index when we get specs from buildcache

Now that only one index file per mirror needs to be fetched in
order to have all the concrete specs for binaries available on the
mirror, we can just fetch and refresh the cached specs every time
instead of needing to use the '-f' flag to force re-reading.