Arvados

Copy load_config.rb and config.rake from API server. Use arvados_config.rb as starting point to write sso_config.rb.

1. Get clusterwide config (should use arvados-server config-dump tool to get it).
2. use ConfigLoader to merge/migrate legacy config
3. update the code to use the new config structure
4. update documentation about using config:check, config:dump and config:migrate to update SSO configuration (documentation covers same for API.)