prune: tolerate all forms of broken slapos.cfg in instance
When we recurse in instance to find nested slapos that might be using parts from ourselves, we check all files named slapos.cfg but we may encounter some slapos.cfg that are not actual slapos config files, for example in testnodes we have a working copy of slapos repository which contains a stack/slapos.cfg which is completly different.
This was breaking prune with an error like this:
2020-04-29 20:07:28 slapos[2097] DEBUG Reading config at /srv/slapgrid/slappart9/srv/testnode/czp/slapos-repository/stack/slapos.cfg
2020-04-29 20:07:28 slapos[2097] ERROR No section: 'slapos'