repo: Add unconfigured-state to remote config options

repo: Add unconfigured-state to remote config options

This is a migration from the origin version.  It's
nicer to have it in the remote, since that's what one
needs to change.  Then tools don't need to mess with
the origin file.o

In fact in this scenario one can keep the "media source" like
`file:///install/repo` or whatever, since conceptually that's where it
came from.  We're just providing a better error.

Closes: https://github.com/ostreedev/ostree/issues/626
Closes: #627
Approved by: jlebon