Test if config (file) parsed with yaml_load() contains a valid Mapping
object, otherwise Patroni throws an explicit exception. It also makes
the Patroni output more explicit when using that kind of "invalid"
configuration.
``` console
$ touch /tmp/patroni.yaml
$ patroni --validate-config /tmp/patroni.yaml
/tmp/patroni.yaml does not contain a dict
invalid config file /tmp/patroni.yaml
```
reportUnnecessaryIsInstance is explicitly ignored since we can't
determine what yaml_safeload can bring from a YAML config (list,
dict,...).
