Dockerise cqlshlib tests

The cqlshlib tests are the last remaining tests that don't run in docker.

Before 4.0 the cqlshlib builds are hardcoded to python2, but the test docker image doesn't support python2.

(Maybe this is why cqlshlib tests don't exist in pre 4.0 circleci?)

Dockerising the cqlshlib tests, makes it a lot easier to keep jenkins' agents clean of zombie processes.

Patches:

• cassandra-builds: https://github.com/apache/cassandra-builds/pull/50
• 3.11: https://github.com/apache/cassandra/compare/cassandra-3.11...thelastpickle:mck/dockerise-cqlsh-tests/3.11
• 3.0: https://github.com/apache/cassandra/compare/cassandra-3.0...thelastpickle:mck/dockerise-cqlsh-tests/3.0

Patches are not required for 4.0 and trunk, as the --no-site-packages flag to virtualenv has already been removed from them.