SSSDConfig: Do not fail with nonexisting domains/services
    
    dict.keys() returns iterator in python3 and not list
    Chaging data in dictionary while using iterator
    fails with "RuntimeError: dictionary changed size during iteration"
    
    https://fedorahosted.org/sssd/ticket/3107
    
    Reviewed-by: Michal Židek <mzidek@redhat.com>
    (cherry picked from commit 1773fdad2730f3f910782781fa286f402ce36cca)