Make /etc a writable mount

Make /etc a writable mount

First, move deployments to /ostree/deploy.  Having them in the
toplevel clutters the filesystem layout too much.

When we deploy a tree like /ostree/deploy/NAME, there is now also a
writable directory /ostree/deploy/NAME-etc.  This is mounted as
read-write inside the system.

On an initial install, that directory is copied from
/ostree/deploy/NAME/etc.  On subsequent deployments, we find any
changes made in the current deployment's /etc, and apply that set of
changes to the new deployment's /etc.

See https://live.gnome.org/OSTree/EverythingInEtcIsABug