Fixes #6448 - cr/update CH with type CH BZ1114065

creating or updating a content host with the type set to 'content host'¬
causes the validation for candlepin to fail because type must be one of¬
the following: system hypervisor candlepin.
ideally 'system' would be a CONSTANT on the System class.  Would you mind updating it to be a constant (along with the other types)?