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.
Status Job ID Name Coverage
  External
failed SlapOS-Master.UnitTest-Master

03:59:12

passed SlapOS.Eggs.UnitTest-Master.Python2

00:33:11

failed SlapOS.Eggs.UnitTest-Master.Python3

failed SlapOS.Eggs.UnitTest-Master.Python2

00:11:39

passed SlapOS.Eggs.UnitTest-Master.Python3

00:14:22

failed SlapOS.Eggs.UnitTest-Master.Python3

00:35:19

failed SlapOS.Eggs.UnitTest-Master.Python3

failed SlapOS.Eggs.UnitTest-Master.Python3