fixes #2360 loading parameters on host edit form when hostgroup or environment is changed
fixes #2417 - SSO abstractioning

Apache and Signo SSO reworked to a new general SSO concept.

You can use SSO service that comes with Katello for loggin in. It's
based on OpenID protocol with slightly customized provider.
fixes #2407 - Add STI to settings model

Also adds some tests for rendering new/missing STI models, and
refactors the old default_settings loader into the new sub-models.