In-tree build scripts

Bring the artifact/deb/rpm build scripts (and associated docker images) from cassandra-builds repo to the .build directory.

The declarative Jenkinsfile can then directly declare the artifacts jobs in its pipeline. And the packaging jobs can be separated and run in parallel. EDIT: this bit has been moved to CASSANDRA-18594

This addresses the epic's stated existing problems:

• difficult to pre-commit test jenkins and cassandra-build changes,
• CI development efforts is split between ci-cassandra and circleci, despite ci-cassandra being our canonical and non-commercial CI,
• lacking parity of what is tested between ci-cassandra and circleci
• cassandra-builds as a separate repo (without release branches matching in-tree) adds complexity to changing matrix values (jdks, pythons, dist)
• mixture of jenkins dsl groovy, declarative and scripting pipeline.
• different pre-commit and post-commit jenkins pipelines are used.

In addition it addresses:

• stage jobs don't always running on the same SHA as the pipeline's run,
• infra issues around networking, specifically git cloning additional cassandra-builds repository,
• a more readable Jenkinsfile
• more UX friendly build and test scripts