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)