rpm: Run external scriptlets on uninstall only when available
    (bsc#1196514 bsc#1196114 bsc#1196942).
    
    When dependency cycles are encountered package dependencies may not be
    fulfilled during zypper transaction at the time scriptlets are run.
    
    This is a problem for kernel scriptlets provided by suse-module-tools
    when migrating to a SLE release that provides these scriptlets only as
    part of LTSS. The suse-module-tools that provides kernel scriptlets may
    be removed early causing migration to fail.
    
    Signed-off-by: Michal Suchanek <msuchanek@suse.de>