Make Razor::Config not a singleton; store it in a class variable in Razor instead

Having Razor::Config as a singleton makes testing very awkward. Since we
want people to refer to the config via Razor.config anyway, there's no real
gain from having it as a singleton, just pain when trying to test it.
Razor::Config: successfully load even empty configs

The missing validation of empty configs will follow

Fixes https://github.com/puppetlabs/razor-server/issues/52
Razor::Config: validate the facts.blacklist
Razor::Config: validate repo_store_root
* config.yaml.sample: remove unneeded repo_service_uri setting
* spec/config_spec.rb: fix test failure

The test 'Razor::Config/validating/repo_store_root/should require that
repo_store_root is set' was failing because at the point where the validate
helper tried to get the key for hte current test, it had already been
deleted.