admin: Rework /ostree/deploy to support multiple independent operating systems

admin: Rework /ostree/deploy to support multiple independent operating systems

The real vision of OSTree is to "multiple versions of multiple
operating systems".  Up until now, it's worked to install gnome-ostree
inside a host distribution, but several things don't work quite right
if you try to do completely different systems.

In the new model, there's the concept of an "osname" which encompasses
a few properties:

1) Its own /var
2) A set of trees deployed in /ostree/deploy/OSNAME/
3) Its own "current" and "previous" links.

Now it no longer really makes sense to boot with "ostree=current".
Instead, you specify e.g. "ostree=gnome/current".

This is an incompatible change to the deployment code - you will need
to run init-os gnome and redeploy.

All "ostree admin" subcommands now take an OSNAME argument.