Jenkins pipeline small fixes: slack notification body, StabilityTestDataPublisher, archive cassandra-test-report.txt

1) The slack notification bodies contain an array of SHA commit messages involved in the build. The array needs to be formatting.
See https://the-asf.slack.com/archives/CS6CA748J/p1592029643136500

2) Remove cassandra-builds directory in Summary stage, so 'Restart from Stage' builds work. If the pipeline gets into the final Summary stage it will clone the cassandra-builds directory. Once this happens it is not possible to restart the pipeline. Restarting from a given stage is useful if one of the test stages failed for unknown/infrastructure reasons.

3) Archive the cassandra-test-report.txt artifact. This plaintext report was added under CASSANDRA-15729. It should also be made easily available as a downloadable artefact on the build.

4) Add StabilityTestDataPublisher to the pipeline junit results. This provides the flakey info next to each test on the test results page. The individual test stages had this enabled, but not the pipeline. For example here: https://github.com/apache/cassandra-builds/blob/master/jenkins-dsl/cassandra_job_dsl_seed.groovy#L252