refactor(sct.py): improve cloud options
refactor(sct.py): improve cloud options

- Add `@cloud_provider_option` decorator
- Add `CloudRegion` param type
- Rename command `prepare_aws_region` to `prepare_region`
- Add `cloud_provider` option to `prepare_region` command
- Get regions only if they are required

scylladb/scylla-cluster-tests,scylladb/scylla-cluster-tests,scylladb/scylla-cluster-tests,scylladb/scylla-longevity-tests,scylladb/scylla-cluster-tests,scylladb/scylla-cluster-tests,scylladb/scylla-longevity-tests,scylladb/scylla-longevity-tests