Add the vault service in the deployment stack

this comes with a few refactoring:

• entrypoint has been improved to:
  • provide a 'celery-worker' command: start a swh worker ($SWH_WORKER_INSTANCE)
  • replace the generic command by a 'rpc-server' command (same behavior as the previsouly default command),
  • add a 'scheduler' command dedicated to running scheduler listener and runner services.

• moved some common env var definitions to env/common-python.env

• mirror.yml has been modifed acordingly.