cli/prune: fix a case where parts where not detected as used from a recursive instance

To consider if a shared part is used, we consider all files named `slapos.cfg`
in the instance and try to parse them as a slapos config file.
The problem was that as soon as a file can not be parsed as a slapos config
file, we did not continue to evaluate other `slapos.cfg` files. It was a
`return` where it should have been a `continue`.

This was especially incorrect, because when we have recursive slapos, we
usually have a working copy of slapos repository, which contain
`stack/slapos.cfg`, which is not a valid slapos config file.

As a result, when a top level shared part was used in a software installed
in the recursive slapos from a theia or slaprunner instance, this shared
part was not detected as used and could have been deleted.

The fix consist in checking all `slapos.cfg` files. This will make slapos
node prune command probably much slower, because it will really iterate on
all files now. Anyway this was fast because it was wrong.