deploy: Return staged deployment
Today rpm-ostree has some code to run a "sanitycheck" on a deployment.
I had initially deleted that when adapting it to use the staging code,
but I realized it should work fine; we just won't see the merged
config, but that's OK.
When I readded that code it started crashing because we didn't
actually return the new deployment object. We'll gain some coverage
here as I'll land the code to have rpm-ostree use staging, then bump
the rpm-ostree tests here.
Closes: #1559
Approved by: jlebon