- KFetConfig `set` method now takes model instance instead of their pk
for this type of entries.
- Add tests for KFetConfig class: tests against types currently used by
kfet app (duration, modelinstance, decimal).
- These tests are located in `kfet/tests/test_config.py`. We should
separate tests suites by file like this, considering what they are about.
- KFetConfig method `set_many` renamed to `set` (because it also updates
only one).
