Deployments: flush the Redis cache before running drush.

This PR is to avoid  a race condition state between DB and cache, which causes features recreation to fail during international deployments. See #3211.

#### What's this PR do?
- Brings new step into `ds build` that clears Redis cache **before** running any `drush` command
  - All cache is flushed during US deployments
  - Only country-specific during international deployments

#### What are the relevant tickets?
Fixes #3211.

Not sure if `redis-cli` alone will connect to the proper (remote) redis server on prod web heads.