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>