bin/compose: Expose phases as [install, postprocess, commit] cmds

[cgwalters]

Right now rpm-ostree compose tree is very prescriptive about how things work.
Trying to add anything that isn't an RPM is absolutely fighting the system. Our
postprocessing system enforces no network access (good for reproducibilty, but
still prescriptive).

There's really a logical split between three phases:

• install: "build a rootfs that installs packages"
• postprocess: "run magical ostree postprocessing like kernel"
• commit: "commit result to ostree"

So there are two high level flows I'd like to enable here. First is to allow
people to do arbitrary postprocessing between install and commit. For
example, run Ansible and change /etc. This path basically is like what we have
today with postprocess-script.sh, except the builder can do anything they want
with network access enabled.

Going much farther, this helps us support a "build with Dockerfile" style flow.
We can then provide tooling to extract the container image, and combine
postprocess and commit.

Or completely the other way - if for example someone wants to use rpm-ostree compose install, they could tar up the result as a Docker/OCI image. That's now
easier; an advantage of this flow over e.g. yum --installroot is the "change
detection" code we have.

Related issues/PRs:

• Add a way to run a script outside of the ostree root after build #96
• demonstrate server-side ansible/chef/etc runs #471

One disadvantage of this approach right now is that if one does go for
the split approach, we lose the "input hash" metadata for example. And
down the line, I'd like to add even more metadata, like the input rpm repos,
which could also be rendered on the client side.

But, I think we can address that later by e.g. caching the metadata in a file in
the install root and picking it back up or something.

[cgwalters]

This was fixed by #1039