sudo: get timezone information from previous value when constructing new usn
    
    Steps to reproduce:
    1. Run sssd with openldap server
    2. Set smart refresh interval to 1 second for faster testing
    3. When no new rules are found, the timezone information is missing
       when creating new usn value
     > Issuing a smart refresh of sudo rules (USN >= 20190919090710)
     instead of
     > Issuing a smart refresh of sudo rules (USN >= 20190919090710Z)
    4. As a consequnce, filter is incorrect when used with openldap server
    
    Resolves:
    https://pagure.io/SSSD/sssd/issue/4085
    
    Reviewed-by: Alexey Tikhonov <atikhonov@redhat.com>