UTIL: add find_domain_by_object_name_ex()
The _ex version of find_domain_by_object_name() has a additional option
'strict'. If set to 'true' NULL is return instead to domain from the
first argument. This way the caller can see if the provider object name
really contains a known domain.
Related to https://pagure.io/SSSD/sssd/issue/3579
Reviewed-by: Fabiano FidĂȘncio <fidencio@redhat.com>