stages/ostree.deploy: create ostree_commit_deploy function

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.

test/data/manifests/fedora-ostree-image.json

@@ -1071,7 +1071,6 @@
           "type": "org.osbuild.ostree.deploy",
           "options": {
             "osname": "fedora",
-            "ref": "fedora/x86_64/osbuild",
             "remote": "osbuild",
             "mounts": [
               "/boot",

test/data/manifests/fedora-ostree-image.mpp.yaml

@@ -58,7 +58,6 @@ pipelines:
       - type: org.osbuild.ostree.deploy
         options:
           osname: fedora
-          ref: fedora/x86_64/osbuild
           remote: osbuild
           mounts:
             - /boot