ncm-spma: retry full update when userpkgs are not allowed

This is to work around the Require/Remove yum failures that can occur
i'm not in favour of modifying `update_pkgs`. don't forget, it works most of the time (like 95+%; small changes in packages to install or no changes at all). the `spare_deps` method can't do a proper resolve because repoquery is stupid (failing to pass the recursive flag) and slow (serial resolv library). dnf should fix both, so there's hope that the retry block is not needed at all in future.
so this else block really is the exception.

however, it can break the deployment (and in particular in the ks-post-reboot when large changes to the installed packages typically occur, and a failed spma results in no direct access to the machine).