cqlsh: add tests for INSERT and UPDATE completion

I've added tests for cqlsh completion for INSERT and UPDATE statements. In the process, I also refactored trycompletions and removed old datatypes from the keyspace initialization script.

The changes are on GitHub here:

https://github.com/mambocab/cassandra/tree/cqlsh-completion-tests

The diff for review is here:

https://github.com/apache/cassandra/compare/trunk...mambocab:cqlsh-completion-tests

Happy to prepare a patch; let me know what to do.