Replace certbot with mod_md for ci.bigtop.apache.org

[Luca Toscano]

Hi folks!

I added an idea in https://issues.apache.org/jira/browse/BIGTOP-4038
about how to automate the let's-encrypt cert renewal for
ci.bigtop.apache.org. Let me know what you think and if you like the
idea!

Luca