Initial commit
Remove Spotify- and Helios-specific stuff
Use spotify/cassandra:base as the base image name
Rename directories to match image tags

* cassandra-singlenode -> cassandra (spotify/cassandra)
* cassandra-clusternode -> cluster (spotify/cassandra:cluster)
Fix CMD and Java/Cassandra installation process

* Add Datastax sources for Cassandra.
* Install OpenJDK.
* Set an empty CMD, overriding the "bash" CMD set by ubuntu:trusty.
Install JNA and dsc20 instead of cassandra
Add workaround for chfn failures
Update README