This stage is similar to ostree.deploy, but deploys from a container
image rather than from an OSTree commit by using the `ostree container
image deploy` command. An example stage definition could look like:

```
  - type: org.osbuild.ostree.deploy.container
    options:
      osname: fedora-coreos
      target_imgref: ostree-remote-registry:fedora:quay.io/fedora/fedora-coreos:stable
      mounts:
	- /boot
	- /boot/efi
      kernel_opts:
	- rw
	- console=tty0
	- console=ttyS0
	- ignition.platform.id=qemu
	- '$ignition_firstboot'
    inputs:
      images:
	type: org.osbuild.containers
	origin: org.osbuild.source
	mpp-resolve-images:
	  images:
	    - source: quay.io/fedora/fedora-coreos
	      tag: stable
```

Co-authored-by: Dusty Mabe <dusty@dustymabe.com>

This drains some of the logic out of `main()` into a
`ostree_commit_deploy()` function. Doing this will make it easier
to diff this stage with the recently added `ostree.deploy.container`
stage.

This commit also changes the `ref` in the schema to be optional,
which is a fixup for 3cc733d. We need to make the ref optional because
the ref could come from the user in the toplevel schema or it could
come from input commit in the schema.

This example shows how to build a qcow2 very similar to the one
delivered by Fedora CoreOS. It uses an input container that has
the Fedora CoreOS OSTree already baked into it, which means that
the OSTree isn't built using this manifest, but taken as an input
from a remote registry.