SYSDB: Fix setting dataExpireTimestamp if sysdb is supposed to set the current time
sysdb is already able to retrieve the current timestamp if the caller
doesn't specify it. However, for the timestamp cache this came too late
and the timestamp cache used zero as the 'now' time.
Resolves:
https://fedorahosted.org/sssd/ticket/3064
Reviewed-by: Pavel Březina <pbrezina@redhat.com>