Modify the `config_combos()` to return `repo_configs` and `root_dir`
only if it should be really used. Otherwise, return `None`. Modify all
helper functions for dnf-depsolve API calls to add relevant fields to
the request JSON, only if the relevant values are set. This makes the
test cleaner, since previously, the `root_dir` was always set.
The same applies to `dnf_config`, which could be set to `None` already,
so let's make it optional.
Signed-off-by: Tomáš Hozza <thozza@redhat.com>
e635a8a067