Support /etc/ostree/remotes.d

For many OS install scenarios, one runs through an installer which may
come with embedded data, and then the OS is configured post-install to
receive updates.

In this model, it'd be nice to avoid the post-install having to rewrite
the /ostree/repo/config file.

Additionally, it feels weird for admins to interact with "/ostree" -
let's make the system feel more like Unix and have our important
configuration in /etc.

https://bugzilla.gnome.org/show_bug.cgi?id=729343