SSSDConfig: Do not raise exception if config_file_version is missing
Ticket:
https://fedorahosted.org/sssd/ticket/2837
Reviewed-by: Lukáš Slebodník <lslebodn@redhat.com>
(cherry picked from commit 6a044fa43d53638c1d0b874d43f58c0428820362)
(cherry picked from commit a2363aa5984a707b8834816ea8538fe7de250a63)