Add note mentioning VAULT_NAMESPACE variable to vault docs

What does this MR do?

If users are following this doc step by step with an HCP Vault instance, they'll encounter some errors stating the roles do not exist with the example CI job. This is because HCP Vault employs a default namespace that must be defined via the VAULT_NAMESPACE environment variable.

This has led to a handful of tickets, so this MR simply adds a note mentioning this variable must be present if HCP is being used.

Related issues

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