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.

And you were right, US production cli client isn't connected to the server.
@blisteringherb and I have discussed possible ways out.

@mshmsh5000 What would you say if we skipped this step for US?
We are not sure if US prod is affected at all so it might make sense to just run the code only on intl servers.

CC @aaronschachter