accept plan_details from service gateways

Instead of sending just a list of plan names, the service gateway can
now send an array ("plan_details") containing details about the service
plans
This commit adds the "extra" attribute to service plans
Expose ServicePlan#extra
[bugfix] don't filter "free"ness
Refactor legacy_service_gateway_spec.rb
Make "rake spec" the default rake task
[finishes #46157255] Service offering's plans attributes are whitelisted for safety
ServicePlan's #create calls bypass mass-assignment verification