lib/deploy: Do post-ops when removing staged commit

lib/deploy: Do post-ops when removing staged commit

These are further fixes based on running more of the rpm-ostree
test suite.

When dropping the staged deployment, we do need to do the
"post operations" such as bumping the sysroot mtime, so that
clients know something changed.  We also need to regenerate
the deployment refs.  And of course do a sysroot reload.

Also, add a "base cleanup" after creating a staged deployment
which also regenerates the refs.

Closes: #1570
Approved by: jlebon