DNFPayload: blivet.size.Size() only knows 'spec' kwarg now.
Payloads: make DEFAULT_REPOS a part of the interface.

Source spoke knows about its existence since 069cb536. The change makes
the DNFPayload work with the spoke too.
DNFPayload: do not crash when an addon is unavailable.

Log & disable the addon, it's what YumPayload does.
DNFPayload: Add languageGroups().
dnf_img_test
running-dnfimg-localy
DNFPayload: apply the kicstart excludedList.