CB: Create schema and upgrade scripts for Cassandra
This ticket covers updating Cassandra schema to facilitate Config Backend feature for 1.5.0 release:
https://gitlab.isc.org/isc-projects/kea/wikis/designs/configuration-in-db-design#cassandra
I adds new tables, scripts and upgrade scripts to Cassandra.
To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information