Add "unconfigured-state" concept to origin files

Some operating systems may come with external tools for subscription
management that drive access to the content.  In that case, the origin
file may not be useful (for example, it could refer to an installer
ISO).

This patch will allow OS installers to inject that state, with a
useful error message, directing the system administrator to an
external tool.

See: https://github.com/projectatomic/rpm-ostree/issues/31

https://bugzilla.gnome.org/show_bug.cgi?id=737686