sysroot: Add a flag to suppress post-deploy cleanup

sysroot: Add a flag to suppress post-deploy cleanup

I noticed seeing the output of `prune` twice in rpm-ostree, and had
always wondered why.  When reading the rpm-ostree code to fix
something else, reasons, I noticed the reason - we were pruning once
here, and then once after rpm-ostree regenerates its "base" refs.

There's no reason to clean twice, so let's add a flag so rpm-ostree
can suppress doing it inside libostree.

Closes: #474
Approved by: giuseppe