sudo: use proper datetime for default modifyTimestamp value
    
    The current default was simply "1", however OpenLDAP server was unable
    to compare modifyTimestamp attribute to simple number. A proper datetime
    is required by OpenLDAP.
    
    It worked correctly on 389-ds.
    
    Steps to reproduce:
    1. install openldap server
    2. run sssd
    3. there are no sudo rules on the server and there are no cached objects
    4. you'll see in the logs that sudo smart refresh uses `(&(&(objectclass=sudoRole)(modifyTimestamp>=1))...` filter (`1` instead of proper datetime value)
    
    The minimum accepted value by OpenLDAP is 00000101000000Z, as both month and day can not be zero.
    
    Resolves:
    https://pagure.io/SSSD/sssd/issue/4046