Make creation of /etc/zypp/repos.d entries optional

[lkocman]

Leap 16.0 is going to use openSUSE-repos for repo management by default, so repo definitions created by Agama are in our case redundant. Same story is for Leap Micro 6.0 which was already released. Originally requested here https://code.opensuse.org/leap/features/issue/91

The benefit of this ability to fetch sets of new repositories as part of migration https://en.opensuse.org/SDB:System_upgrade_to_LeapMicro_6.0#2._Fetch_new_repositories_and_new_keys and avoid any sort of manual sed/awk replacement, repo renames, removals, additions .... that used to take close to 50% of the migration wiki page

The package itself maintains entries under /etc/zypp.repos.d/(openSUSE:|NVIDIA:).repo

We do have a rpmsave / archiving section for default repo definitions in %post, however it would be nice if we would not create these in the first place.

https://build.opensuse.org/projects/Base:System/packages/openSUSE-repos/files/openSUSE-repos.spec?expand=1