add verb to do container finalization

[cgwalters]

This is split out of discussion on ostreedev/ostree-rs-ext#159

Basically since we have a ton of "postprocessing" logic in rpm-ostree around things like tmpfiles.d etc. and rpm-specific postprocessing like the rpmdb normalization, we should add e.g.:

rpm-ostree ex-container commit

(Feel free to bikeshed naming)

That would basically do all of that stuff.

[cgwalters]

Also there's https://github.com/cgwalters/fcos-derivation-example/pull/1/files#diff-dd2c0eb6ea5cfc6c4bd4eac30934e2d5746747af48fef6da689e85b752f39557R14
So we should have rpm-ostree cleanup -m work in a container too.

[jmarrero]

This was done on: ostreedev/ostree-rs-ext#205