WIP: Add a Cassandra Demo

[wallrj]

Release note:

NONE

[jetstack-ci-bot]

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[jetstack-ci-bot]

@wallrj PR needs rebase

[munnerz]

Can this be closed, or otherwise be turned into a quick start doc and published in our docs? Looks like useful info if we don't already have it, but I think should be more visible than in demo.sh 😄