Setting up Cassandra database service on TravisCI

At this time of writing TravisCI says that if you want to run a Cassandra service you have to add a cassandra service according to https://docs.travis-ci.com/user/database-setup/#cassandra:

services:
  - cassandra

but if you try to initialize cassandra you might find out that cassandra is not yet ready or running depending on timing.

The solution is:

• a .travis.yml file (see https://github.com/davidemoro/pytest-play-docker/blob/master/.travis.yml) with the cassandra service and a "bash travis/setup_cassandra.sh" command in before_script section
• a travis/setup_cassandra.sh (see https://github.com/davidemoro/pytest-play-docker/blob/master/travis/setup_cassandra.sh) that waits for cassandra up and running (e.g., a sleep 30, a polling loop with max retries implemented in my travis/setup_cassandra.sh script or using docker service) before cqlsh initialization commands

And now cassandra is ready to be used in your tests (for example https://github.com/davidemoro/pytest-play-docker/blob/master/tests/test_cassandra.yml . In this case I'm using using plain yml files thanks to pytest-play)