Use Ubuntu-20.04 for vpp-csit-* job executor images

Use Ubuntu-20.04 for vpp-csit-* job executor images

- refactor vpp-csit job names to conform to the vpp
  job nomenclature
- add ubuntu-20.04 for CSIT image builder install
  requirements
- fix executor class verification
- add option to build_executor_docker_image.sh to
  dump Dockerfile without building image(s)
- refactor Dockerfile generation to support
  executor classes
- add support for csit_shim and csit_dut docker
  image generation from csit ansible repo
- refactor docker installation
- refactor locale support
- remove old static csit Dockerfiles
- remove unused jenkins labels
- use csit-builder-{os}-prod-{executor_arch} label for
  vpp csit device test
- run tox in csit_pip_cache() to include pylint
  requirements in python cache to avoid downloading
  from pypi during job execution.

Change-Id: I5f4ffc587c6b8c43cb874dfd8c615ce56d3b97a4
Signed-off-by: Dave Wallace <dwallacelf@gmail.com>