ipa: use mpg aware group lookup in get_object_from_cache()
    
    Since with algorithmic id-mapping SSSD automatically creates user
    private groups for AD user with the help of magic private groups (mpg)
    get_object_from_cache() should use mpg aware calls to make sure the
    right user object is found when handling a request to look up a user
    private group.
    
    Only the lookup by gid had to be modified because
    sysdb_search_group_by_name() used for lookups by name is aware of MPGs.
    
    Related to https://pagure.io/SSSD/sssd/issue/3748
    
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>